Reloading Press: 5.56x45mm Mk318 mod 0

The Reloading Press is going to be a new short feature for me in 2016, based on a few comments from friends of mine in other media.

Each post will contain a cartridge and stats for GURPS – sort of. The way I’ll be doing this is to use my ballistics calculator, which will give gameable and consistent stats.

In many cases, there won’t be much difference between various versions of a bullet or cartridge. As an example, at some point I’ll compare 115gr ,124gr, and 147gr 9mm ammunition. That will, within GURPS’ resolution, likely not amount to much.

5.56x45mm Mk318 mod 0

This cartridge is the USMCs answer to the fact that the 62gr M855 (about 4 grams, and 940m/s out of a 20″ barrel) didn’t pereform that well when fired out of the usual short-barreled M4 carbine, which has a 14.5″ (370mm) barrel standard.

The ammo is built to a 2 MoA standard, which means that the maximum accuracy a weapon can get using this ammo is Acc 5 (about 1.8 MoA, close enough).

Pertinent stats for the ballistic calculator:

M318 mod 0
Chamber Pressure 59950 psi
Barrel bore 5.7 mm
Case Length 45 mm
Chamber Bore 9.29 mm
Barrel length 355.6 mm
Bullet Mass 62 grains
Aspect Ratio 4.1 L/Bore
Burn length 11.1 mm
Projectile Caliber 5.7 mm
Total Accelerated Mass 62 grains

Output Stats

 As designed, the 14″ barrel produces about 5d damage. We’ll see how that changes as a function of barrel length.
1/2 D: 406 yds  – The range at which drag slows the bullet to half the velocity at the muzzle. This is a function of bullet construction and not barrel length or initial velocity using my model.

Performance as a function of Barrel Length

The minimum penetration for this round out of a 4″ (102mm) barrel is about 3d. So we’ll start there, going up in barrel length and velocity.
Some notes:
  • The velocity is at the muzzle, listed in meters per second. I have mixed units in this chart in a horrible unforgivable way. I’ll fix that in the next post.
  • The pi to pi- range is where the damage drops from piercing to small piercing. For very short barrels (smaller than 128mm in my model), the bullet is slow enough that it will not yaw and fragment, and so it’s only good for pi- rather than pi. This is defined as 600m/s in my model, which is somewhat arbitrary, if valid for the older 5.56x45mm rounds like the M193.
  • It’s possible that the damage (penetration, really) is perhaps 10% high, but the alternate construction and higher velocity out of shortr barrels of this particular round might actually rate the increased penetration
  • The Mk318 is designed to be barrier blind and to provide reliable expansion and fragmentation via construction. See Tactical Shooting p. 77 for Barrier Blind ammo rules.
  • The slightly higher pressure I used won’t cause undue wear in barrels (the 63,000psi used for the M855A1 does cause such wear; I’ve seen estimates of up to 50% reduction in barrel life, and since the chambers of M4 type weapons are rated to only 62,000 or 63,000 psi, the cartridge itself is pushing its luck. But I digress.) and if the design pressure is actually still 55,000 psi (I couldn’t find a reference), the higher velocity out of 14″ barrels is likely a result of a bit more powder, or more energetic powder.


The Mk318 can be fired out of pretty much any weapon chambered in 5.56x45mm – which means every M16, M4, Steyr AUG, L85, and too many variants of standard military rifles to count. Unlike the more specialized rechamberings required for other ammunition, this one is just buy it, load it, and shoot it.This was the first round treated in this manner, but more will come. I will definitely do my personal favorite, the 6.8x43mm SPC, and also the 6.5 Grendel, plus some more pistol and rifle rounds. I’ll definitely take requests!

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