The 6mm SAW cartridge. A great general purpose combat cartridge the US military designed, but never adopted
Since writing my original General Purpose Combat Cartridge Article. I’ve done more research, more thinking and learned a LOT.
But most importantly, I’ve also shed some assumptions I didn’t realize I had.
In the original article, I only looked at bullets between 6.5mm and 7mm which weighed between 120gr and 130gr. That’s because the US military decided in the 1920s and early 1930s that was the ideal size for a military bullet.
But what if it’s not?
Remember, the military was using only full metal jacket (FMJ) bullets. Traditional FMJ bullets pretty much suck at killing things compared to modern options.
Therefore, a better designed bullet could be smaller and still be effective.
So I expanded my horizons. I tried smaller bullets and played with the numbers a little bit to find a better general purpose combat cartridge. The results floored me and made me almost giddy for the better part of a day.
Click Here for some nifty drop-down text with a summery of the previous article In the original article I made two basic assumptions about our general purpose combat cartridge: The Hornady Hits Calculator uses a bullet’s diameter, weight, and velocity to give a rough estimation what you need to kill game animals. It’s not perfect, but it’s a very useful tool. 500 HITS is considered enough for deer. The standard military 308 loading is called M80 ball. It’s a 147 Grain, .308 caliber bullet with a 2800 fps muzzle velocity. It retains 1277 fps at 800 yards, which gives it a HITS score of 415. I decided to make the target a little higher at 450. In the end, I decided a 6.5mm bullet @ 2600 fps would be a Great choice for a a general purpose combat cartridge. The 6.5mm at 800 yards hits about like a .357 magnum hits at point blank range. (a 4″ barrel 357 with 130gr bullets comes out at 1450-1500 fps) Anyway… Here’s the updated/expanded chart. (These values were created using a G7 BC assuming a form Factor of 1.0. With a form factor of 1.0, then G7 BC = sectional density. The bullets all have a Sectional Density around .261. +/- .001) First, a 92gr 5.56 bullet @ 3180 fps would burn out a barrel so fast it would never work as a general purpose combat cartridge. But look at the 6mm bullet. It has the least drop, least wind drift, and the least recoil while still being more lethal than our current 147gr 308 load. Did I mention almost HALF the recoil? Compared directly to the 5.56 (with 20″ barrel), the 308, and our previous 6.5mm cartridge, we get this: Our 6mm bullet easily beats both the 308 and 5.56 in Every Single Category except muzzle energy, where it only loses to the 308. Compared to all of them, the 6mm option has 3 feet less drop. It beats the 5.56 in wind drift by almost 4 feet and the 308 by almost 2 feet, though it beats the 6.5mm option by only 6 inches. Compared to our previous 6.5mm general purpose combat cartridge, the 6mm option wins in drop, drift and most importantly recoil. However, the 6mm loses slightly – about 11% – in long range lethality. However, the 6mm option has a higher hit probability due to less drop and wind drift. That fact alone makes me pick the 6mm over the 6.5 because Shot placement is King. The most important factor in stopping an enemy attacker is shot placement. Less drop and drift make shot placement easier. Many people (including myself) would argue that the 6mm bullet is more lethal because shot placement (the most important factor) is easier. And you can’t miss fast enough to win a gunfight. The 6mm has less recoil which means faster follow up shots in close quarters combat. Plus, higher velocity which is a huge help against armor. (speed defeats armor better than high mass.) I don’t think a theoretical 11% increase in lethal potential is enough to justify a very real 23% increase in recoil, a 21% increase in drop, and 12% increase in wind drift…. Especially when the 6mm bullet is lethal enough. The 6mm bullet is already more lethal than the 308 at 800 yards if you go by the numbers. To give you a sense of scale, 800 yards is over 6 1/2 football fields away (including the end zone) and our little 6mm bullet could kill an enemy soldier at that range. To be clear, I don’t think a 108 grain 6mm bullet at 1600 FPS (800 yard velocity) will hit an enemy soldier like the Hammer of Thor. It simply can’t. But it can hit like a .357 Magnum hits at point blank range, and feel free to double check me on that. A lightweight ~110 grain .357 magnum can reach in 1500-1650 FPS range. Some will point out that the .357 magnum is a much bigger bullet so it will do more damage. That’s (sort of) right in theory, but wrong in execution. What matters is what a bullet does and most importantly, WHERE it hits. A properly constructed 6mm bullet can do everything a .357 magnum bullet of equal weight and velocity can do. Emphasis on “Properly constructed”. I’ll talk more about proper bullet construction in a minute. In the meantime, do we agree that a .357 Magnum at point-blank range is VERY deadly? We do? Good. Moving on. I’m not the first person to realize the potential of a 6mm general purpose combat cartridge. The United States Navy is. “Contemporary medical reports of the day noted the 112-grain (0.26 oz; 7.3 g) bullet produced noticeably greater damage to tissue and bone than other military cartridges of the day when fired at full velocity (2,560 fps)” Note: the “cartridges of it’s day” included the powerful .45-70 cartridge. However, it didn’t do much damage unless it yawed. (turned sideways/tumbled) And since it used a round nosed bullet, it rarely yawed. It was truly ahead of it’s time… And that’s what killed it. The 6mm Lee Navy was invented before the modern Spitzer shape bullet and it’s round nosed bullet lost velocity very quickly. Even so, it was considered to have an effective range of 500-700 yards. (depending on who you read.) Additionally, the metallurgy of the time couldn’t handle the small bore combined with high pressure. This was made worse because smokeless gunpowder was still quite new. The powders of the day made the barrel wear problem worse. Ultimately, the Navy dropped the 6mm Lee Navy in 1899. Back to the Future Pt 2 A 6mm general purpose combat cartridge was considered again by the United States in the 1970s. It was called the 6mm SAW and it fired a 105 grain 6mm bullet at 2520 FPS. The performance was great, but the military didn’t want a 3rd cartridge in it’s inventory. Eventually, 6mm SAW project was cancelled when a new and improved 5.56 round was promised. This new round arrived as the M855. The new M855 didn’t make the 5.56 more lethal, and contributed to the 5.56/223’s reputation as both a great and awful cartridge. The lack of technology killed a 6mm general purpose combat cartridge the first time, and money killed it the second. Back to the Future part 3 will feature some horses, a train, and a modified Delorean. Just kidding. 😉 Now, lets talk about bullet construction. One of the most important factors that’s ignored by the Hornady HITS calculator is the type of bullet used. Quality Hollowpoints are best, but they’re expensive and against the Hague convention. Fortunately, there are several alternatives that are cheap enough and effective enough to be seriously considered. M855 Cross Section M855 (62gr “Green Tip” Light Armor piercing) The M855 is the current standard issue bullet to our military. It consists of a steel penetrator sitting on top of a lead core and surrounded by a copper jacket. To be clear, I don’t think the M855 construction should be used exactly as configured because the M855 bullet has two significant problems. Problem #1: The jacket is too strong. The M855 relies on mostly on fragmentation to deal it’s damage. However, the copper jacket is much stronger than it needs to be. This means higher velocities are needed to make it fragment which reduces effective range. Problem #2: There is no design mechanism to ensure the bullet yaws (turns sideways/tumbles). Quite simply, the bullet relies on fragmentation, but there’s nothing in the design to ensure it fragments. This makes the M855 inconsistent at best in combat. When it does what it’s supposed to, the M855 is amazing. However, it doesn’t do it consistently. Mk 318 cross section. MK318 Mod 1 SOST The bottom half of the Mark 318 bullet is basically a solid copper slug, while the top half is a conventional Open Tip Match (OTM) Bullet. They have reasonable accuracy, fragment at high velocity, and the solid copper base penetrates through barriers with enough force to cause damage. However, Fragmentation is VERY velocity dependent. Out of a 14.5 barrel, the current M855 bullet only fragments well out to 60-150 yards. The MK318 Mod 1 SOST, even at near 3000 fps muzzle velocity, only fragments well out to 200-250 yards. Because of the low effective range, I prefer a bullets where fragmentation isn’t required to be effective. There’s at least one design that does that quite well. The 7N6 bullet has a hollow cavity in the front of the bullet which deforms when it hits the target. This causes the bullet to destabilize and start tumbling almost the instant it makes contact. The tumbling bullet creates significant cavitation and a suitably impressive wound channel. Additionally, a large part of the bullet is a steel core which allows it to punch through armor with relative ease. It doesn’t fragment like the 5.56 bullets do, so the 5.56 creates more impressive wounds at close range. However, the 7N6 wounds are effective, especially when you consider they’re being done with a 53 grain bullet. Imagine what a 108 grain bullet could do… Additionally, because of the air pocket in the nose, the 7N6 bullet will start tumbling almost instantly at virtually all ranges. Since we are talking about an 800 yard combat range, being effective at ALL ranges is VERY important. The problem with the 7N6? No Fragmentation. While it tumbles almost instantly, it won’t fragment. Like ever. So Here’s my solution: Combine the M855 and 7N6 into one “super bullet” that’s still Hague convention legal. And by that I mean keep the basic M855 design (full copper jacket, steel penetrator in front of a lead core) But add the air space in the nose of the bullet like the 7N6 to ensure the bullet will tumble and fragment. At close range it will fragment causing a devastating wound. At long range it will tumble and still cause significant damage. Plus, the steel core penetrator will allow our soldiers to shoot through most common barriers the bad guys hide behind. It would be an amazing bullet for our general purpose combat cartridge. Longer bullets with thinner jackets fragment at lower velocities. A 108 grain 6mm bullet is already quite long, and will be longer with the air space and steel core. I would make the jacket as thin as possible while still ensuring the bullet will hold together in flight. This should extend fragmentation range as far as possible. As an alternative, the Mk 318 design could be used. That would simplify construction and possibly be cheaper on a per-round basis. Also, because it requires fewer components it might be easier to make accurate. So the next obvious question is: What cartridge should fire this 108 Grain 6mm bullet at 2830 FPS? 6mm BRX (Bob Crone’s Version) There are two cartridges currently available to give us a working base cartridge. The 6mm BRX (Bob Crone’s version) and the 6mm Dasher. Either can propel a 108 grain bullet to at least 2830 FPS from a sensible 20″ barrel. (I’m assuming a bullpup design to make a 20″ barrel practical) Both cartridges use a standard .308 Winchester case diameter (.4728″) and both have an overall length of 2.44 inches. Both are almost exclusively used by the target shooting community and noted for EXTREME accuracy. Accuracy is obviously needed for an 800 yard general purpose combat cartridge. However, the needs of a military cartridge are different that a target shooter. First and foremost, barrel life is a consideration. The enemy of barrel life is pressure. In order to maintain velocity with less pressure, you need more powder and a larger case. If you move the case shoulder and neck forward a little, you can get that extra room. Going from the 6 BR to the 6 BRX involves moving the shoulder .100 and gains about 200 FPS velocity. So another .100 should lower the pressures significantly with the same velocity. (and then lengthening the neck to at least .25 to properly hold the bullet) The lower pressure plus modern technologies like nitride treated barrels should extend barrel life to equal or exceed the 308. Cartridge Weight. Looking at the weight of 6BRX brass plus our lengthened case gives us a ballpark weight of 131 grains for the case. The bullet is 108 grains, the primer is about 6 grains and the powder charge would probably be around 35 grains. (again looking at the 6 BRX numbers) Added together, that gives our 6mm a weight of 280 grains per round. For comparison, the 5.56 M855 weighs 183 grains per round and the 308 M80 weighs 393 grains per round. For ammo weight, that gives us: (not including magazines) To look at it another way, our soldier carry 210 rounds of 5.56 M855, or 5.49 pounds of ammo. For that same weight our soldiers could carry: (rounding up 0, 3 & 2 respectively) Because this is a low pressure cartridge, a steel case could be used. That would make it slightly lighter than a brass case. Better still, the 6mm SAW successfully used Aluminum cases which would make our 6mm cartridge really light. (only ~45 grains for the case, 194 grains for the total) EDIT: A company called Shell Shock Technologies has created a Nickel-plated Aluminum cartridge that is supposed to withstand 65,000 PSI. Since our cartridge will probably only be ~50,000, it should work perfectly. That would make our total cartridge weight roughly 200 grains. That’s only 10% more than the current 5.56 ammo. Plus, they are cheaper to make than conventional brass cases. But the fact remains our 6mm General purpose combat cartridge means few rounds on a per-soldier basis. Having fewer rounds isn’t exactly a desirable thing. However, I will make this argument: I’ve talked to a lot of rank and file combat veterans from Iraq and Afghanistan to as far back as Vietnam. The vast majority say the light recoil and large amount of ammo carried with the 5.56 allowed (almost encouraged) them to “spray and pray”. And I’m not talking about suppressing fire either. That’s both Army and Marine Corps soldiers. (yes even the Marines spray and pray) You can’t miss fast enough to win a gunfight. Period. Our little general purpose combat cartridge would double the recoil and reduce the number of rounds carried. perhaps that would be incentive to help our soldiers slow down and actually aim their weapons. To back that up, our soldiers used an estimated 250,000 bullets to kill a SINGLE enemy insurgent in Iraq and Afghanistan. Yes, that’s two hundred and fifty THOUSAND bullets for a single enemy kill. In Vietnam where there was a mix of the M14 and M16, the ratio was one-fifth of that at 50,000 bullets per enemy kill. In WWII, it was around 20,000 bullets per enemy kill. I partially attribute this to better marksmanship training and partially to heavier recoiling weapons. Regardless, less “spray and pray” shooting is a good thing. Our little 6mm cartridge would require a new rifle because it’s too long and wide for an AR15 action. I believe a bullpup design is necessary to get a barrel long enough to get the needed velocity while still being a practical overall length. Someone would need to design that rifle, and I’d love to be the guy. I have been designed a bullpup with CAD software for over a decade. Currently it weighs only 5 1/2 pounds empty, is completely ambidextrous, has a 20″ barrel, and is only 26 1/8″ inches long (without muzzle device.) It also has a TINY parts count, (comparable to a Glock) and can be field stripped in mere seconds with no tools. I designed it from the ground up as a general purpose combat rifle. As to the rifle optics. I recommend a low powered variable scope in the 1-6 power range like the Trijicon VCOG. At 1 power, it functions much like a reflex sight. At high power you can accurately make hits out to 800 yards with a proper reticle. In my mind, the best reticle available is the 1-6x Advanced Combined Sighting System (ACSS) reticle from Primary Arms. I would also consider the reticle in the Trijicon VCOG, though the ACSS Reticle is FAR more advanced and useful. If I had my druthers, I would take a Trijicon VCOG with the Primary Arms 1-6 ACSS reticle (First Focal Plane of course). In my mind, that would be the perfect scope for a general purpose combat cartridge. The truth is, I’ve actually been selling our little 6mm general purpose combat cartridge short this whole article. No, seriously. That’s because I’ve been using a G7 Form factor of 1.0 (If you don’t know what form factors are, the chief ballistician of Berger Bullets Brian Litz wrote a wonderful article on Form factors. You can find it Here) Short version: the better the form factor, the more aerodynamic the bullet. (Lower is better) I’ve been using a form factor of 1.0 for our 6mm bullet. However, other military bullets have much better form factors. For example, the 7N6 bullet has a Form Factor of .920. That’s significantly better than 1.0 I’ve been using. So if I recompute with the more aerodynamic form factor, the results look like this: Yeah, the difference isn’t mind blowing. But over 80 FPS difference at 800 yards is pretty great. Plus 6 inches less drop and 5 inches less wind drift. Not bad for our little 6mm general purpose combat cartridge.General Purpose Combat Cartridge Round 2
Bullet Diameter Bullet weight Recoil (ft-lbs) Muzzle Velocity 800y Velocity 800y Drop 800y Drift Muzzle Energy 800y Energy 800y HITS 5.56mm 92 10.1 3180 1866 -133.7 40.7 2066 711 450 6mm 108 8.5 2830 1600 -176.2 49.4 1920 614 450 .257 120 10.6 2837 1602 -176.1 49.5 2144 684 450 6.5mm 127 9.4 2510 1362 -235.1 60.4 1776 523 450 6.8mm 140 9.3 2330 1232 -279.6 68.0 1688 472 450 Cartridge Bullet weight Recoil (ft-lbs) Muzzle Velocity 800y Velocity 800y drop 800y drift Muzzle Energy 800y Energy 800y HITS M855 62gr 3.8 3112 1105 -213.9 94.1 1333 168 121 6mm GPCC 108 8.5 2830 1600 -176.2 49.4 1920 614 450 6.5mm GPCC 130 10.5 2600 1450 -213 55.3 1951 607 501 M80 Ball 147gr 15.8 2800 1277 -215.2 72.8 2559 532 415 
A Note On Bullet Lethality
Back to the Future
In fact, it was officially adopted by the Navy in 1895 and saw service in the Spanish American War. It’s called the 6mm Lee Navy and it fired a 112 grain 6mm bullet at 2560fps.Bullet Construction
The 7N6 bullet of the 5.45×39.The Cartridge
A Quick Word About the Rifle and Optics
The “Secret Sauce”
Cartridge Bullet weight Recoil (ft-lbs) Muzzle Velocity 800y Velocity 800y drop 800y drift Muzzle Energy 800y Energy 800y HITS 6mm
1.0 FF108 8.5 2830 1600 -176.2 49.4 1920 614 450 6mm
.920 FF108 8.5 2830 1683 -169.6 44.5 1920 679 474
Just for the record, but the US never signed the Hague Convention and nether did the terrorists or many third world nations.
So the rules of the Hague Convention don’t apply to us or them.
Also, how did you not know about the M855A1 when it was issued in 2010. Do you live under a rock? People were blogging left and right about that round before it was even done.
The new round is designed to tumble because the weight is towards the back, the issues you mentioned with green-tip were fixed years ago. You are complaining about something that was already fixed.
No the US didn’t sign the Hague convention, but they stubbornly abide by something they haven’t signed. I have no idea why.
I don’t consider the M855A1 a serious contender because it has… Issues. It’s MUCH more expensive than other options, that alone is enough to disqualify it. It has other problems, but cost alone disqualifies it.
At http://www.g2mil.com/6mm_optimum_cartridge, a guy named Stan crust proposes a 6mm optimum cartridge which he describes as the magnum version of the 70’s 6mm SAW. Using a .25 Remington case necked down to accept a .24 caliber 100 grn projectile with a muzzle velocity of 2900 fps.
We came to (almost) the exact same conclusion. 🙂
The only problem I see is the parent case he selected. The .25 Remington can only push a ~100 grain bullet at 2300-2400 without pushing the pressure. Like I said in the article, higher pressure means shorter barrel life so that’s a no-go. Other than that, we definitely agree on the solution.
Abe, what are your thoughts on using the .220 Russian as the parent case. Your solution is a 7.62 NATO parent case that will stack 20 bullets in a standard mag. The .220 Russian should stack 25 in a standard mag, where the 5.56 NATO will stack 30 in a standard mag. The 6.5 Grendel has had great success with the .220 Russian parent case and the .264 USA (6.5mm) is using an elongated .220 Russian parent case in the LSAT and CTSAS programs. I was thinking that a 6mm in a .220 Russian elongated case might be the best way to go for bullet stack in a standard mag. Also, I was wondering about the .30 Remington parent case of the 6.8 SPC. This should stack 28ish in a standard mag. Possibly a 6mm in an elongated .30 Remington parent case? Your thoughts on both of these solutions would be great.
I also think that the future US Rifle should be an ambidextrous bull pup for CQB and long range engagements. I notices that you jumped straight to 20 inch barrels on your 6mm solution. My other question is…how would your solution work on a 16 inch barrel, a 14.5 inch barrel and a very small 10 inch barrel for CQB in a very small bull pup like the Israelis are using. Are there advantages to 6.5mm and 6.8mm over 6mm in 16, 14.5, and 10 inch barrels?
The .220 as a parent might work, the problem is pressure. If you look at the 6mm Fat Rat (6.5 Grendel necked down to 6mm), you’ll notice they get good velocity with 105-108 grain loads, but they need long barrels and high pressures to do it. Lengthening the OAL to 2.5 might relieve this problem, but I’m not sure if you can get 2800 FPS with a 20″ barrel *and* do it with relatively low pressure. (around 50,000 PSI) if it works, it would probably be a better solution than a .308 as a parent.
If the .220 Russian is marginal, then the .30 Remington is even harder because it’s smaller. I’m not saying it’s not possible, but I don’t think it’s likely. I know it could be done using the .473 rim of a 308 because of the 6mm Dasher and 6mm BRX.
There’s an inverse relationship between bullet diameter and velocity. The wider the bullet, the faster it can go for a given amount of powder/pressure. So a wider bullet can use a shorter barrel to get the same velocity, but it sheds velocity too fast to be useful.
As for Barrel length, I picked 20″ because longer barrels allow for more velocity with less powder. 20″ seems to be the sweet spot for diminishing returns. The Firearm blog did an interesting article that talks a little about the weight problems of shorter barrels. For me, It’s a pure performance issue. A 20″ barrel in a bullpup is easy to get under 30 inches total length, which is ideal for urban/vehicular operations. It also allows for lower port pressures which reduce port erosion and allows for reduced wear on the operating components.
Abe,
I really appreciate your practical view on a GPC. I served, as a UK battlefield aviation pilot, in Northern Ireland, Bosnia and 15 months in Iraq. In the process, I heard as many opinions on the subject of the 5.56 as there were soldiers to express one.
We, with the L85A2 (the A1 was terrible but the A2 pretty close to awesome for a first Gen Bullpup, if a bit heavy), had fewer problems than the Americans with the 5.56. The L85 gave us a full length barrel in a carbine length weapon, helping retain that key facet of 5.56 effectiveness, velocity. That said, it still showed it’s shortcomings and a bullpup platform has serious limitations in urban CQB, which is why the UK’s SF used the Canadian C8 while I served with them, and then the HK416 just after I moved on.
It’s with this considered that the 5.56 becomes a serious liability. With carbine length barrels, (16″ or less), the 5.56 is only marginally more effective than a classic submachine gun round, and significantly worse than a 7.62×39, which is why the 6.8 SPC appeared. From 0-600m, the SPC is head and shoulders above the 5.56 as it’ll perform out of a carbine. However, from 400+, it suffers from the classic 7.62 problem, the ‘short fat draggy bullet’ syndrome and could not be a GPC because it’s 600-1000m ballistics get really bad, really quickly.
A 6.5×43 based on the same case and loaded to a COAL up to 2.45″ would allow the 130gr bullets you discussed in your first article, have similar case capacity and performance to the 6.5 Grendel. Importantly, that would deliver an 800yd HITS of 450 out of a 16″ carbine barrel.
With most of the world’s population being urban, we won’t get away from a 16″ barrel because any Bullpup that can swap RH to LH ejection in the time it takes to sprint across a back alley, would cost a fortune. I really like the L85A2 and would rather pack it in open country over an M16/M4, but I was much happier with my HK53 in urban areas, despite the lack of energy it’d deliver.
Such a cartridge would require a new platform, but that’s long overdue anyway. I like the short stroke gas piston of the AR-18/L85/G36 series and that basic platform would be easy to re-design for a 2.5″ cartridge. Either that or the Galil ACE platform, which being an AK derivative, is pretty simple to modify for a different COAL, and about as reliable a platform as you’ll find anywhere.
As for the ammo weight issue that I hear Americans talk about all the time, us Brits have a different view. We re-issued the GPMG at the fire team level due to the range/effect problems with the 5.56 in Afghanistan. A fire team with three carbines and a LMG, all in 6.5×43, would be carrying more rounds for the same weight than they currently do. All good for us ‘special relationship’ allies.
Keep up the work, the naysayers will eventually break and the last thing we need is a return to the 0.30 calibre in any of its potential forms. Teaching a young soldier good marksmanship is much easier with half the recoil of the 7.62 and there’s no such thing as ‘more dead’. We need a cartridge that will do the job and little more. Too much is just that, too much.
My brain is too tired after work to read the whole comment and all I saw was you mention that a bullpup that could swap ejection fast would cost too much but Kel Tec has the solution and honestly it should be the future of bullpups (hopefully from some more reputable and higher quality companies)
Downward ejection! Fully ambi, no need to swap anything, no eating brass, which for a lefty shooter like me is a massive plus. But personally I dislike bulpups for their ergonomics, triggers and strange balance, so I’d rather have the 16″ carbine barrel or 18″ SPR barrel AR15 chambered in 6.5 Grendel and just accept the velocity loss from the shorter barrel. Truth is most of us civilians aren’t ever gonna be engaged in any kind of combat and if we ever were it would definitely not be extending past 600-800 yards. But for the military they should probably listen to the R&D department more and really get a good effective combat rifle and cartridge down pat and soon, we’ve wasted WAYYYYYYYY too much time and money on this to keep going in circles.
Agreed the Military needs to get it’s act together and decide on a great cartridge/rifle combo. However, I don’t think it’ll be downward ejection for a long time because Kel-Tec patented the downward ejection system on it’s RDB. We’re looking at 15-20 years (depending on when they filed the patent) before another company can do it. And I just don’t see the military adopting the RDB.
The military could always license the technology from Kel-Tec. I’m sure George Kelgren would jump at the offer.
Forget the Kel-Tec, because…well…Kel-Tec. Have a look at the Desert Tech MBR ambidextrous bull-pup. I think we may have a winner here. Plus the Kel-Tec that ejects its rounds behind and below the magazine…I think that really limits clever future carrier bolt designs.
How about an ejection system similar to the FN F2000 to adress the bullpup problems with urban combat?
It’s a decent solution, but not ideal because it’s necessarily complicated. The Kel-tec RFB does it like that too and the Desert Tech MDR does something similar. (the Military Arms Channel has a video that shows the system) I would prefer the system in the Kel-Tec RDB where it goes down, but that limits the how short the rifle can be.
For my own rifle, I’m working on a a system that allow you to switch ejection sides with the push of a button. I have a lot of design work to go before it’s a reality, but that’s the goal.
How would or does a .243 Winchester compare with what your looking for in the 6mm cartridge? It would fit all .308/7.62 NATO equipment and has 60+ years under its belt.
It would work if it was loaded correctly, but it’s physically too large. The larger the cartridge the longer and heavier the action must be. All of the guns currently chambered for 308/243 sized cartridges are too long and heavy. That said, I do like the 243’s performance. It’s a very versatile cartridge for anything but larger game.
It is known for a long time that holy grail of high bc bullets is between .243-.284″
Both .257 and .270 have not been explored in US for such bullets and .243 would be
really on the light side of the knockdown power.There is no way that military would use those crazy long 6mm bullets in service rifles.This leaves us with 6.5 and 7mm.
Both with great bullet selection but 7mm in intermediate case would be limited to
154 grain and lighter and shorter bullet.It clearly comes back to 6.5 mm caliber
from 100 to 144 grain bullets.The only way that such change would make sense
is if you can also make it work for basic service rifle,MG and squad marksman rifle.
Only standard case of intermediate length has the capacity to go to heavier
6.5 bullets.Grendel case is severely restricted with longer bullets so far into the case
and 6.8 case would only work with high bc bullets in .257 if they made some around
.500 bc otherwise it does not have the case capacity for 6.5 bullets and .270
bullets currently used are low bc and weight for caliber.
What about a bullpup like the desert tech mdr? In a 6.5 or 6mm? Also can it be done in <20in barrel. I think 16 is a great overall but understand that may be too short to achieve velocities. But every inch matters (thats what she said) imho in cqb. Also I agree with the brit. Find a cartrige you can use in carbines/ rifles and support weapons lmg/mmg. Everyone carry the same ammo.
The MDR is… Okay. It’s heavy at 8.67lb (3.93kg), only has a 16″ barrel at 26.2″ OAL, and the price tag is north of $2k. The MDR also has a HUGE parts count. Seriously, it’s well north of 150 parts (roughly ~50% more than the AR-15) To quote Scotty from Star Trek “The more they overthink the plumbing, the easier it is to stop up the drain.“. I’ve taken apart enough old military weapons and spent enough time with CAD to know you don’t need that many parts.
26.1” overall (without muzzle device) with a 20″ barrel is doable. Not easy, but doable. It took me years to crack the design, but I finally figured it out. (CAD software is God’s gift to designers btw). And if you need the gun shorter, you could go to a 16″ barrel and only be 22.1″ overall. (And as a pint of pride, my rifle design has less than 50 parts total.)
Abe,
Quite apart from your own current view on best calibre/cartridge for GPCC, here’s a more general question on the same subject:
Given a hypothetical situation where both gun and cartridge can be designed from scratch (meaning without restrictions deriving from equipment already in use), what, in your personal opinion as a gunsmith, would be the maximum total cartridge length for a short action bullpup design to work?
Background to my question: I’m thinking the Grendel case is too short for high bc-bullets/stanag/approx2900fps and .243 win too long as you yourself state. Why not _something-akin-to_ the 6.8 spc case (smaller diameter case base than the Grendel; stacks more rounds in a mag) necked down to whatever favourite calibre 6-6.5mm and let a long bullet stick out more than the 57.40mm total.
It depends on what you mean by “short action”. Technically, the 30-06 length actions are considered “long action” as opposed to the .308 length actions which are often called “Short action”. By contrast, the 5.56 length actions I would call “carbine length action” or something like that. There are .308 bullpups, so the action can me made as long as the cartridge requires.
Regardless, for a cartridge to perform at that level, 2.5″ OAL (~64mm) should be plenty. It could probably be done even shorter (I’m not a cartridge tech). The .224 Valkyrie is close to a 6.8 necked down to 5.56 and it does very well. For 6mm, you’d need a little more space but not much. Again, 2.5″ should be plenty and it could probably be done even shorter.
Abe, have you or anyone else done fine-grade calculations around 6mm to determine where the true optimum point is? (It’d be pure numerical fluke if it were *exactly* 6mm as opposed to 6.1 or 5.92 or some such.)
Second question: Would “blown forward” case-shoulder designs be problematic in semiautomatic weaponry? (I bring this up because the 6mm Grinch wildcat that won an event last year had such a desigh, but as far as I recall, it was a bolt-action benchrest competition.)
I suspect it would be a range of numbers instead of an exact one. (5.56-6 for example) Ideally, you want the bullet to be as small and light as possible to get the job done. But you can’t go too small or you won’t do enough damage or be aerodynamic enough to reach the target.
Probably the lower limit is around the size of the .224 Valkyrie cartridge (90gr @ ~2700 from 24″ barrel) Much smaller just doesn’t work. The upper limit is about 6mm (108gr @ ~2800) because of recoil. The trouble is it depends on other factors (like barrel life) that aren’t easy to calculate and depend on a yet more factors.
You’ve got my brain spinning on how to plot all the various factors on a graph though. Hmm…
I love your article, Abe. Thanks for giving the GPC concept the attention it deserves 🙂
In my own fiddling, I looked for the optimal weight / BC trade-off which would maximize the range at which velocity drops below 2600 feet/second for a 6mm bullet from a 6mm AR cartridge with the same shape as Hornady’s A-MAX.
Why 2600fps? Because that’s the minimum velocity at which FMJ bullets (of any weight) offer significant fragmentation, and Fackler agrees with you wrt the relationship between fragmentation and terminal effect.
The sweet spot with those criteria was 87gr, quite a bit lighter than the bullets you’re using in your analysis. Any lighter and they lack the BC, any heavier and they lack the muzzle velocity. On the upside, using a bullet that light would help close the weight-per-round gap vs 5.56x45mm.
As you point out, there are better-fragmenting bullets than FMJ, but alas that is the only composition I know how to model for velocity-vs-fragmentation.
Thanks again for the interesting reading 🙂 please write more!
We are seeing a common theme here with your 6mm, the 6mm SAW and the Russian 6mm Universal. The U.S. is now working on several 6.5mm projects at once in order to match or better the effective range of the 7.62x54R with talk of an effective range (and implied lethality?) of 1200 meters with a view to replace both 5.56 and 7.62. One track seems to be a high tech, longer term ideal solition with cased telescoped technology. The other track seems to be a short term, low tech solution with the .264USA and an AR derivative, perhaps to hedge their bets if we go to war with Russia and China (God forbid) before they can field the first option. Concurrently, there are 2 PDW programs being undertaken, one by SOCOM and one for a 9mm by the Big Army . It seems clear where the U.S. is headed regarding small arms procurement and I think it makes sense. It is the direction I would go, even in the selection of a 6.5mm over a 6mm as it seems to give a medium machine gun a bit more “ass”, as it were, for taking out hardened targets; which reminds me of another new Army program to field a tungsten AP round, in 7.62mm no less (incase WW3 kicks off tomorrow?).
Questions:
Do you think the 6mm still makes more sense given the apparent goals of the U.S. military?
Why do you think the U.S. and Russian militaries are so resistant to an ambidextrous bullpup. I have never used one so I don’t see what they are seeing. Is it because it is too hard to clear a malfunction? So make the receiver cover flip open with a latch. Heavy? So lighten it. The handicaps seem to be getting addressed but the RFPs are being written to exclude bullpups (Ref. Primary and Secondary podcasts.) I’m not understanding something here.
I can’t remember my other questions at the moment.
Great blog, by the way. I wish I had seen this years ago. You should guest post on The Firearm Blog.
For a short term “off the shelf solution” I would go with the 224 Valkyrie if it proves to have decent barrel life. You’d need new barrels bolt and mags. Drastic increase in capability with minimum fuss/cost.
My argument for 6mm vs 6.5 is as much related to performance as it is to weight.
First, the 6mm can do everything a 6.5 can in terms of combat effectiveness. With a quality bullet, a ~108gr 6mm bullet can kill out to a long range with good shot placement. Again, at 800 yards the 6mm GPCC hits like a 357 Magnum at point blank range. How much more do you need? (Also, the 6.5mm recoil is too high if you want a good trajectory)
Second, as Nathaniel from TFB so often points out, weight is a HUGE issue with military rounds. In order to get a reasonable aerodynamic 6.5mm round, you’d need 130+ grains of bullet. The extra powder/bullet/brass weight would drastically reduce the number of rounds per pound for very little advantage.
Third, consider that the average rifleman’s hit percentage in combat at 300 yards is less than 10% and tapers to negligible at 600 yards. Going to 6.5mm wouldn’t increase effective damage very much and would only help a small portion of our forces. (Squad marksmen and LMG)
In regard to sub-guns, I just don’t see the point. A bullpup 6mm GPCC could be the same size/weight/capacity but far more capable. Ditching the sub-gun would simplify supply lines too (fewer guns/armorers/ammo types)
There’s a great poem by Sam Walter Foss entitled “The Calf “Path” that perfectly explains why people don’t like bullpups… and there has yet to be a really good one come to the market. They’re all too heavy or not ambi enough. They’re getting better though.
If quick change barrels were made standard on all platforms that use the new general purpose cartridge such that they can be swapped out as a “disposable” item, much like brakes on a car, and that were written into the service contract, would that change the sort of pressures you would design into your cartridge? Would it change caliber?
A note on cost. The more it costs, within reason, the better. Not for us, the tax paying citizens, but for the big defense contractors (Lockheed, General Dynamics, etc.) who serve as the vehicle for the banking cabal to suck more money out of the public using the government as its muscle. You mentioned the cost of the general purpose ammunition and weapon systems as a concern. In your analysis, spend like a drunken banker.
It might change the pressures (maybe) and it might make me look harder at a 224 caliber round with a ~90gr bullet. However, I’m still leaning toward the 6mm. 6mm seems to be the Goldilocks range for recoil vs aerodynamics. Larger bullets get a lot of recoil fast and lighter bullets either can’t hack it or would burn barrels too fast when driving heavy enough bullets to high enough velocities.
And yes, Politicians do seem to be great at aspending our money. Reminds me of the old joke: Politics is contraction of two words: Poly mean “many” and “ticks” meaning bloodsuckers.
Wow, thanks for your posts, glad I found you. I understand maybe 1/2 of what you are talking about, but it is helping me learn. I have a question for you, or anyone that would like to chime in. I like to play at being a bit of a prepper, not super serious, but I do think there are serious possibilities for SHTF. I have been thinking about an off the shelf rifle that would serve both a practical and “fantasy” SHTF roll. I want something that I could hunt with, and something else that could be used for a sniper roll if need be.I also want to take some long range rifle classes, because the idea of being able to hit a target at 1000+ yards really appeals to me. My budget is limited, so a fairly common round is better. For real life hunting, long range target shooting and a far fetched shtf sniper duty, it sounds like you think 6mm Creedmoor or 6.5 Creedmoor might be your choice over .308. Am I understanding you correctly? And I am basing that mostly off the assumption that the ballistics of the round are basically the same, the Creedmoor is just a longer cartridge than the Grendel, from what I gather. Does this sound reasonable? Thanks!
If I had to pick one cartridge for everything except combat, the 6.5 Creedmore would probably be my pick. Ballistically, it spanks the 308 at every range and has less recoil taboot. The chief ballistician of Berger Bullet wrote a paper entitled What’s wrong with 30 caliber that should dissuade you from selecting anything larger than 7mm for long range shooting.
If combat was included, I would probably pick either the 6.5 Grendel of 224 Valkyrie. (If you’re on a tight budget, the Valkryie can even sub for the 6.5 Creedmore because they have somewhat similar trajectories) However, in a SHTF or TEOTWAWKI scenrio I think 5.56 is your best bet because of how common the ammo is.
What do you think about necking down a 6.5 Grendel to 6mm?
@ John: If you look at the 6th comment from the top, I do talk about it a little.
“However, in a SHTF or TEOTWAWKI scenrio I think 5.56 is your best bet because of how common the ammo is.” …
… and therein lies the problem. There is the pursuit of the ideal, and that’s not a bad thing, but for many of us, myself included, we’re already heavily invested in the .223/5.56 NATO and .308/7.62 NATO paradigm – both in weapons purchased, and in ammo stockpiled. My son and I live under the same roof, and between the two of us, we own 8 weapons chambered in 5.56 NATO, and 5 weapons chambered in 7.62 NATO; and we have THOUSANDS of rounds stockpiled in both calibers for those weapons.
Until your proposed 6mm GPCC cartridge has not only been adopted for mil-spec, but also has been around and in mil inventory long enough to be as common available as the 5.56 and 7.62 cartridges are, there is little incentive for the commercial buyer to being purchasing weapons and ammo in the GPCC caliber. God knows how many rounds of 5.56 and 7.62 are CURRENTLY in mil inventory, but has to be in the billions of rounds. And politicians being the asshats that they are, they’ll never order .mil to dump that inventory on the commercial market to make room for the new caliber because that would be too much like arming civilians to suit the antis. And it would be an enormous environmental headache to try and destroy the existing inventory and recycle the components. So although it may not be optimum, I think the two existing calibers are here to stay for quite a while longer.
When I was a kid (I’m 66 right now), I remember going to gun shows at the local American Legion Hall, where you could buy mil-surp .30-06 in bulk, by the bushel basket full. That was around the early 1960s, the M14 was the nation’s primary battle rifle, and .308/7.62 the cartridge. The political climate was much different then. There were a lot of M1 Garands and ‘03/A3 Springfields in private hands, not to mention .30-06 chambered hunting rifles, and gun ownership and marksmanship programs were much more part of the popular and cultural mainstream than they are today, with all the crazies out there trying to wipe out the 2nd Amendment.
As the M16 rifle began to be phased in with its new cartridge, there was no need to dump mil-surp 7.62 on the market because it was still in use for the M-60 and later M240 machineguns….which continues into the present. The political climate today is such that it seems unlikely that gov’t would order .mil to dump billions of rounds of ammo onto the commercial market to make room for its new GPCC chambered weapons. Heck, we’ve had politicians in RECENT memory to blocked the re-importation of old Garands from Korea for American collectors, and it’s not even a “modern” military rifle.
I know it’s a long winded reply, but I think my point is that you’re not going to be able to get the military to be the first adopter of this cartridge, using their interest as a gateway into the commercial market. It will simply have to be introduced in the same manner as the 6.5 Creedmoor was – as a commercial offering that fits a niche and is a solution to some problems not adequately addressed by other calibers. It will have to stand on its own in the commercial market place, before .mil will begin to take any possible interest in it. And if/when .mil begins to take an interest, it will likely be through the agency of the various services’ marksmanship teams, which will bring it to the attention of the spec ops community. But just because spec ops takes an interest, that’s not a guarantee of success.
We have .300 Blk because spec ops found that it filled a need, and they took interest in it. But I own a 10” SBR in .300 Blk – pretty much the ideal length for the caliber – and while it may be serviceable in competition with the 7.62×39, I’m honestly NOT as impressed with the subsonics as you say you are. I like shooting them suppressed, because it is nice and quiet, but shooting a 200+ grain FMJ or HPBT at subsonic velocities gives it no more stank than a +P .45 ACP, and all it does is poke holes through targets. Yeah, shot placement is king, but pain and broken bones counts for a lot too, and I’m guessing that a +P .45 ACP JHP is a more immediate stopper than a subsonic .300 Blk. I’m looking forward to hog hunting with that SBR, but I’ll be shooting 125 grain supersonics, thank you … which will work great, but pretty much negates the advantage of stealth which is what originally sold the cartridge.
But even with the (slowly) growing commercial market for .300 Blk, the .mil community interest in the cartridge has not grown apace. Unfortunately, I fear that this might be the ultimate fate of your 6mm GPCC.
Oi, Abe! Arfcom just dropped a video on the Wolf 100 gr Grendel Military Classic round which totally supports your theory here. At much lower velocity than the M855A1 the bullet into gel effect was quite impressive.
Hi Abe,
Great read…just found your site and am impressed with your knowledge.
That said, I think conflating our current military tactics of fire and maneuver with “spray and pray” when comparing number of rounds expended to kill one enemy in A-Stan/IRQ vs. Vietnam isn’t exactly the best way to analyze the numbers.
F&M has come into a world of its own under the GWOT (hate that term) and the guys at the pointy end of the spear use it quite effectively to kill the enemy, but at an admittedly much higher overall round count.
In short, not all rounds fired in combat are intended to kill the enemy under current combat tactics, but I think we can all agree that more effective caliber/ammunition and a platform that’s slightly bigger than STANAG mags currently allow would be a welcome change.
I wonder your opinion about the latest military trials for 6.8 SPC with offerings from General Dynamics, SIG Sauer, Textron, & PCP Tactical; having polymer, steel and hybrid cases to lighten the effective weight load. https://www.youtube.com/watch?v=M7MV0H8235o
As far as light machine guns go….I’m really surprised the SAW didn’t get dropped in favor of KAC’s LAMG (though preferably not in the 5.56 or .308 calibers) if for no other reason than the weight savings.
Just wondering your take….
I’ve been following the polymer cased ammo a bit and like it for the weight. Probably better than what I suggested, or will be soon. 6.8 is – in my opinion – just too big. it’s nice the new weapons focus on recoil reduction, but imagine if the same technology was applied to a smaller (6mm?) cartridge.
You mention that having your current design eject down limits how short the gun can be. Maybe you need to ask just how short your design needs to be. If it offers the performance of a 20″ barrel in a platform with OAL of an M4 carbine or less while ejecting down, would you consider that good enough?
Also, may want to consider a means of adjusting length of pull. I’ve never fired a bullpup before, but I’ve heard it’s a common problem.
Considering the 6mm ARC cartridge that just dropoped, I would say this article has aged very well.
Hi Abe,
How close do you figure the relatively new Hornady 6mm ARC comes to the theoretical 6mm general purpose combat cartridge you describe here?
It’s a couple hundred FPS slower than my theoretical cartridge, but a significant improvement over the other options out there. The 6.5 Grendel is wonderful, but a touch large. The .224 Valkyrie is good, but a touch small. The 6mm ARC is about the best possible performance you could get from a 6mm bullet out of an AR-15 length action. You’d need a larger action to get the performance I suggested.
That said, I’m very interested in the 6mm ARC. If it gains any significant popularity so the ammo is good and there’s some decent plinking ammo, it’ll probably become my new favorite rifle cartridge.
Hi Abe,
I “think” I see a problem with one of the data points you mentioned above. In the article you state that the 6mm BRX and the 6mm Dasher can propel a 108 grain bullet to at least 2830 FPS from a sensible 20″ barrel.
The thing is… I’m not so sure whether that’s actually an accurate representation of the performance of either of those cartridges (at least not from a 20″ barrel).
While I could certainly be mistaken, I “believe” that even using a bolt gun, you’d need a barrel length of (at the absolute minimum) 24″ in order to get 2830 FPS out of a 108 grain pill using either of those cartridges.
I “think” that in order to hit your goal of having a 108 grain projectile moving at 2830 FPS out of a 20″ barreled (probably gas-operated) semi-auto rifle platform, you’d be forced to step up the power more than a bit (e.g., maybe up to something close to a 6mm GT?). Of course doing so would play havok with your current 6mm recoil calculations, and you’d also certainly need to go with a much heavier/beefier (e.g., AR-10 type) platform to handle it, thereby making such a round impractical for a GPCC.
In addition, I question whether a small chassis (e.g., AR-15 or equivalent) semi-auto rifle platform is appropriate for cartridges as powerful as the aforementioned 6mm BRX or 6mm Dasher. The construction of the AR-15 (or equivalent) upper does not lend itself to those types of full sized cartridges, their required heavier barrels, etc. There is a reason that the AR-10 platform exists, and I think those cartridges are better suited for that type of larger semi-auto rifle platform (or bolt guns).
As of today, I think one of the most powerful 6mm rounds that one can reliably stuff into a small chassis (e.g., AR-15 type) rifle platform, is “probably” the 6mm AR Turbo 40 Improved (ART40I). Anything bigger than that in 6mm, and we’re “probably” starting to get out of AR-15 territory and into AR-10 territory (in terms of semi-auto rifles).
However, even with careful handloading, the 6mm ART40I needs (at least) a 26″ barrel in a gas-operated semi-auto rifle, in order to be able to throw a 108 gr pill down range at your specified target with a muzzle velocity of ~2830 fps. If you were to chop the barrel down to 20″, I don’t think you’d ever be able to get even that round up to your target speed (even with a ton of handload tweaking).
Perchance if you have some data that demonstrates I’m all wrong with my velocity vs barrel length vs cartridge type vs gun type metrics, I’d really like to see them. Many thanks! 🙂
Never mind, I see from the comments above, that it’s already understood that rounds such as the 6mm BRX and the 6mm Dasher aren’t suitable for the smaller AR-15 type action.
However, I still don’t understand your comment about how “the 6mm BRX or 6mm Dasher can propel a 108 grain bullet to at least 2830 FPS from a sensible 20″ barrel”. Based on the resources I have access to, it just doesn’t seem possible to achieve that high of a muzzle velocity with either of those cartridges from a barrel of that length. Again, it appears that the absolute minimum barrel length needed to get 108gr pills up to 2830 FPS using those rounds seems to be 24″. Please note that to get that kind of performance out of them, seems to require very careful handloading (in order to avoid overpressure situations). Regardless, I’d be very happy to be proven wrong. 🙂 Thank you for this wonderful article!
The BRX is Bob Crone’s version, which is different than a standard BRX. The Dasher is capable, but yes it can require a bit of fussing with handloads to get there. If I was talking about use the Dasher as-is, it would be a problem to do it in a military loading. But the idea was the Dasher was a starting point to figure out how large a capable case would need to be. My “ideal” case is a bit longer with different shoulder placement which should allow you to get to those velocities without pushing pressure (which is a no-no for barrel life.)
That said, I’m not an expert on internal ballistics. I might be overestimating the Dasher a bit, but in the end I’m not sure it’ll matter. Like I said, it’s a starting point and needs a capacity increase to hit the desired speed without excess pressure.
Hi Abe,
I actually think the “6mm GT” cartridge (if paired with a shell shock case) actually meets all of the specs you’ve outlined above for the GPCC.
That is to say:
1. It basically has BRX dimensions, but the shoulder has been moved out another ~0.100″, and shoulder angle has also been modified to 35% (vs 30% for BRX).
2. Case neck is elongated to at least 0.25″.
3. Optimal powder charge is reportedly about ~35 gr.
4. Recoil is reportedly slightly more than, but very comparable to, Dasher.
5. Barrel life is reportedly significantly enhanced over BRX / Dasher.
As mentioned in the comments above, getting 108’s up to 2830 fps, out of a 20″ gas gun, with either a BRX or Dasher is pretty much and impossibility.
However, if loaded “really” carefully, I believe you “might” just barely be able to get up to that speed using a 6 GT and a 20″ tube (though I’ve never actually seen anyone attempt to do so). What do you think, is it possible that the 6 GT actually meets your desired GPCC specs?