General Purpose Combat Cartridge: A New Angle

General Purpose Combat Cartridge options

Left to right: .276 Pedersen, .270 British, 7×43 British, 7.62×51 NATO, 7×46 UIAC, and 6.5×38 Grendel

EDIT: I wrote an updated version of this article entitled General Purpose Combat Cartridge Revisited.  Please see that article for updated info.

Of all the General Purpose Combat Cartridge talk that’s taken place lately, I haven’t seen it looked at from this angle before.

(BTW, I define a General Purpose Combat Cartridge as a single cartridge which is effective over the entire range of distances a soldier could engage an enemy. In Afghanistan, the enemy engaged from up to 900 yards away.  Inside 600yds is much more likely, with 800-900yds being a maximum)

I’m going to take the minimum velocity required to be lethal at 800 yards. Then I’m going to work backwards to find the required muzzle velocity.

First, a few assumptions. (that’s usually the weak point in any theory, but I digress.)

  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 gives a conservative estimate of the bullet diameter, weight and velocity required to reliably kill game. It’s not a perfect way to estimate a bullet’s lethal ability.  But should give some “ballpark” numbers to help design our general purpose combat cartridge.

The Hornady HITS Calculator can be found here

(The HITS calculator puts out good numbers, but the simple truth doesn’t line up with any formula perfectly.  My article on terminal performance & stopping power explains how bullets actually do their damage.)

Please keep in mind: 500 HITS is the minimum for deer-sized game.

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.

Lets make our target a little higher at 450.

The military did extensive testing in the 1920-1930s to create a General Purpose Combat Cartridge.

They found that 120 grains was the minimum bullet weight for combat.  They also found bullets heavier than 130 grains didn’t make the bullet more lethal against humans. Lastly, they decided the bullet must be at least 6.5mm in diameter, and no larger than 7mm.

(Yes, we ended up with a 7.62mm (.308) bullet after saying 7mm was the maximum. My article on why the .308 sucks – and the Military knew it, explains why.)

Below is the required velocity to get a 450 HITS score for 120-130 grain bullets between 6.5 & 7mm.  (the .308 is shown for comparison.)

Bullet DiameterBullet weightBullet VelocityHITS Score
6.5mm1201525450
6.5mm1301300450
6.8mm1201680450
6.8mm1301430450
7mm1201760450
7mm1301505450
7.621471277415

Below is a chart listing the required muzzle velocity to get a HITS score of 450 at 800 yards. The ballistic coefficients were calculated using G7 BC assuming a Form Factor of 1 (G7 BC = Sectional Density / G7 Form Factor)

Note that as sectional density goes down, the muzzle velocity goes up very quickly. That’s because higher sectional density bullets don’t lose speed as quickly.  More details in my Sectional Density article.

DiameterBullet weightSectional Density
/ G7BC
Recoil (ft-lbs)MV for 450 HITS
@ 800 Yards
6.5mm120.24610.52812 fps
6.5mm130.2669.02400 fps
6.8mm120.22314.33188 fps
6.8mm130.24211.52706 fps
7mm120.21316.23382 fps
7mm130.23013.92889 fps

In my mind, the clear winner is the 130 grain 6.5mm.

It offers the same HITS score with much less recoil compared to all other options.  Low recoil is very important for a general purpose combat cartridge because of close quarters combat.  Low recoil allows faster follow up shots and quicker sight recovery.

Plus, if you increase the velocity of the 6.5 130gr to a very mild 2600 fps, cool things happen.  Especially if you compare it the current military long range .308 loading. (175gr .308 bullet @ 2580 fps)

LoadingRecoil
(ft-lbs)
800yd HITS
800yd Drop
(inches)
800yd Drift
(inches)
800 Yard
Velocity
130gr 6.5mm
@ 2600 fps
10.5501-21355.31450
175gr .308
@ 2580 fps
18.0621-22963.31344

Remember, 500 HITS is the minimum for deer.  The 175gr 308 is more lethal at long range. No doubt about it.

But the 130gr 6.5mm is lethal enough.

At 800 Yards, the 130gr 6.5mm hits like a .357 Magnum at point blank range. (4″ barrel .357 revolver shooting a 130 grain bullet is around 1450-1500 fps.)

Clint Eastwood Do you feel lucky punk

Yes he had a .44 magnum not a .357 magnum, but you get the idea…

Do we agree that a .357 Magnum at point blank range is deadly?

Good.

The 130gr 6.5mm has also less bullet drop and drifts less in the wind.

Most importantly: The 175gr .308 produces over 70% more recoil than our 130gr 6.5mm @ 2600 fps. 

A General Purpose Combat Cartridge must be effective in close quarters combat.  308’s recoil is too great for that, so it can’t be used. (we effectively have 2 World Wars, plus Korea & Vietnam proving this.  More info in my why the .308 sucks – and the military knew it article)

Our little 130gr 6.5mm bullet at 2600 fps has low enough recoil to be effective in close quarters combat.  It drops less and drifts less in the wind than .308.   Plus, at 800 yards it hits like a .357 magnum does at point blank range.

I think it would make an excellent General Purpose Combat Cartridge.

Do you agree?

Disagree?

Let me know in the comments below.

EDIT: I wrote an updated version of this article entitled General Purpose Combat Cartridge Revisited.  Please see that article for updated info.

P.S. I think you could bump the velocity up another 100 FPS and still keep the recoil low enough for a general purpose combat cartridge. But that would require some (military) testing to confirm.

7 Comments

  1. bantal silikon November 27, 2015
    • Abe November 27, 2015
  2. Pawel Koziarek September 1, 2017
    • Abe (admin) September 3, 2017
  3. Joel November 7, 2017
  4. Pawel Koziarek November 23, 2017

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