Reloading Press: 10mm Auto (10x25mm)

The Reloading Press is an at-least-weekly feature here on Gaming Ballistic for 2016. Each week it looks at some interesting real-world cartridges and presents them with hopefully-useful information in GURPS Format.

10mm Auto (10x25mm)

The 10mm was designed by none other than Colonel Jeff Cooper. Created to be a flatter shooting cartridge than the .45 ACP but hit harder and wound better than the 9mm. When you take a projectile that is substantially heavier than the 9mm, larger diameter, and have the audacity to throw it 100fps faster as well, success is a foregone conclusion.

At least by that metric. Undoubtedly a powerful cartridge – the heavier loads outthwack a .357M – it earns its power by being large, kicking hard, and being generally hard to control. The same problems that exist with large autos in .44M or even the mighty .50AE but on a smaller scale.

The cartridge’s power drew the attention of the FBI in the wake of the famous Miami shootout, and the cartridge was selected as the standard FBI chambering in the S&W 1076 in 1989. The amazing recoil and large pistol size resulted in the creation of the .40S&W a scant year later.

Certain agencies still use the 10mm, and given the round’s power, it’s not a surprise. That being said, if you’re going to load up with an SMG that shoots a 10mm bullet, you will probably get at least as much oomph out of a 5.56mm or 6.8mm carbine as well.
Ballistic’s Calculator Inputs

Basic inputs for the calculator are as follows, selecting 117mm for the (test) barrel length.

180gr 10mm Auto
Chamber Pressure 37500 psi
Barrel bore 10 mm
Case Length 25 mm
Chamber Bore 10 mm
Barrel length 117 mm
Bullet Mass 180 grains
Aspect Ratio 1.8 L/Bore
Burn length 11.6 mm
Projectile Caliber 10 mm
Total Accelerated Mass 180 grains
Expansion Ratio 1.78 expansion
Projectile Load 1

Output Stats

The 10mm Auto is kind of a beast. Sure, it’s not a .44M, mostly. But it’s very powerful, and as there are very few reasons not to carry the most energetic load if you’re carrying one of these at all, I elected to pick more or less the same 180gr bullet used in my .40S&W example, but fired quite a bit faster.


  • The velocity is at the muzzle. The velocity with a 4.6″ (117mm) test barrel is tuned to match real-world data at 1350fps with a 180gr JHP bullet. This is not the highest energy 10mm available, but it’s pretty close. It gives a good comparison to a .40 on steroids.
  • The breakpoint for pi+ in GURPS is currently at 10mm/.40″. You can see that’s a bit generous, as the bullet modeled would be better at closer to +1 per die. Still, because of that breakpoint, 10mm bullets hit a sweet spot.
Some more notes on barrel length. There are really two practical lengths for pistols here. The 3-4″ set will deliver 3d damage. A full-sized pistol with a 4.5-5″ barrel hits 3d+1. SMG barrels between 6.5 and 9.5″ long will get 4d-1. Unlike the .40, the 10mm continues to gain a bit in damage, with a 9.7-14.5″ barrel turning in 4d. A 16-20″ rifle will hit 4d+1, but beyond that you are into very long weapons.

The JHP with modern design is quite impressive, expanding in this case to 1.77x its starting diameter – and there are commercially-available loads that will expand to a full 2x the starting diameter. The standard load with the 1.78x expansion has an amazing calculated wound channel modifier of 2.4x, making the damage with this load out of a 117mm barrel 2d+2 {2.4} pi, or 3d+1 (0.5) pi++ using RAW. That’s considerably more injury out of a handgun than many or even most 9mm SMGs.

Alternate Loads

If you don’t go heavy and fairly fast, you can go light and ridiculously so. One of the more energetic loads is a 155gr projectile at a blistering 1500fps. This doesn’t really change the stats.


For a cartridge that faced very rapid obsolescence at the hands of the .40 “Short and Wimpy”, after the first purpose-built Bren Ten handguns came out, there were a substantial number of quality follow-ons. The Colt Delta Elite and Smith 1076 were big in this chambering, and Glock released their Glock 20 and smaller Glock 29 in 10mm. Both of these were quite large-frame handguns, but in general if you could grip their double-stack .45ACP (the Glock 21), you could hold and shoot their 10mm.

You can get high end firearms in 10mm from such luminaries as Kimber (bout $1,000) or STI ($1600), and these are mostly built on heavy, steel, 1911-style frames.

For SMGs, you seem to have the MP5/10 by H&K, and the MP5/10 by H&K. Seriously. With the right ammo (the fast, light load above) you will still get 4d (0.5) pi++ out of the 225mm barrel, which ain’t all bad.

More pix follow.

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