General Purpose Combat Cartridge Revisited

6mm SAW general purpose combat cartridge

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:

  1. The standard military 308 is considered to be effective to at least 800 yards.  So our cartridge should be as lethal as the 308 at 800 yards.
  2. The Hornady HITS calculator is a decent way to compare bullet lethality.

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

General Purpose Combat Cartridge Round 2

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)

Bullet DiameterBullet weightRecoil (ft-lbs)Muzzle Velocity800y Velocity800y Drop800y DriftMuzzle Energy800y Energy800y HITS
5.56mm9210.131801866-133.740.72066711450
6mm1088.528301600-176.249.41920614450
.25712010.628371602-176.149.52144684450
6.5mm1279.425101362-235.160.41776523450
6.8mm1409.323301232-279.668.01688472450

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:

CartridgeBullet weightRecoil (ft-lbs)Muzzle Velocity800y Velocity800y drop800y driftMuzzle Energy800y Energy800y HITS
M85562gr3.831121105-213.994.11333168121
6mm GPCC1088.528301600-176.249.41920614450
6.5mm GPCC13010.526001450-21355.31951607501
M80 Ball147gr15.828001277-215.272.82559532415

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.

General Purpose Combat Cartridge Trajectory Chart

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.

A Note On Bullet Lethality

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.

Back to the Future

I’m not the first person to realize the potential of a 6mm general purpose combat cartridge.

The United States Navy is.

6mm Lee NavyIn 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.

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.

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 Diagram

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 Mod 0 cross section

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.

7N6 Cross SectionThe 7N6 bullet of the 5.45×39.

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.

The Cartridge

So the next obvious question is: What cartridge should fire this 108 Grain 6mm bullet at 2830 FPS?

6mm BRX Bob Crone's Version

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)

  • 38.3 rounds/pound for 5.56
  • 25.0 rounds/pound for our 6mm
  • 17.8 rounds/pound for 308

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)

  • 210 rounds of 5.56 M855
  • 140 rounds of our 6mm
  • 100 rounds of 308

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.

A Quick Word About the Rifle and Optics

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 “Secret Sauce”

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:

CartridgeBullet weightRecoil (ft-lbs)Muzzle Velocity800y Velocity800y drop800y driftMuzzle Energy800y Energy800y HITS
6mm
1.0 FF
1088.528301600-176.249.41920614450
6mm
.920 FF
1088.528301683-169.644.51920679474

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.

13 Comments

  1. Warhead77777 August 24, 2016
    • Abe August 25, 2016
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      • Marc Stone June 11, 2017
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  3. Stew Staudinger, Sqn Ldr (Ret'd) February 22, 2017
    • Stevean June 8, 2017
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        • Yodude June 9, 2017
          • Marc Stone June 14, 2017
  4. Scott July 10, 2017
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