When Shawn Fisher emailed me that the US Army was looking at adopting a new medium machinegun, this one capable of accurate automatic fire at a range of over a mile, where the M-2 in .50BMG can only deliver single shots, I was intrigued. When I heard it was being considered in .338 Norma Magnum, I knew it was time for a reloading press.

The case and length were designed purposefully to launch the 300-grain, 8.6mm Sierra MatchKing bullet. It’s a long projectile, with an aproximate aspect ratio of 5.6 (compare the 5.56, 7.62, and 12.7mm standard projectiles at about 4:1). This will provide outstanding velocity retention and penetration with distance. The projectile length is an artifact – or more properly – a design intent, of the cartridge and projectile’s intended use: long-range precision marksmanship.

Ballistic’s Calculator Inputs

Based on cartridge dimensions from the wiki, and velocity and barrel length information from the Lightweight Medium Machinegun page.

This cartridge has a very high rated pressure – nearly 64,000 psi. This is even higher than the M855A1 that was known to cause barrel wear in M4 carbines, but lower than many Weatherby Magnum cartridges. Weatherby seems to love loading their projectiles, from the .300 WM to the .460 WM (!!) to 65,000psi.

The projectile’s output predicts a half-damage range of just over 1,000 yards, and a maximum range of 6,275 yards. The real life claimed max range is 6,170 yards, which ain’t bad for my crude model.

Output Statistics

The weapon that this one is designed for is the LMMG, which has a 24″ standard barrel and is designed to launch the 300gr bullet at about 2,650fps (808 m/s). At the muzzle, it will produce 9d+1 pi+ damage, penetrating 0.45″ of RHA out to 1,000 yards . . . at least in GURPS. You’ll hit as hard at 430 yards as a 7.62x51mm does at the muzzle. This is designed around a long barrel, so we’ll start the bidding, so to speak at about 8d of damage (28 points) and go from there.

So while you can’t quite get true .50BMG performance out of the projectile, you’re not too far from it, and you can credibly deliver a threat to Class IV body armor (about 10d) with a ball round from a 31″ barrel, and probably lance right through it with a proper APHC projectile. This is a serious cartridge, and a credible threat.

Platforms

The platform that triggered (hah hah) the design was the Lightweight Medium Machinegun. Designed after hard lessons about long-range firepower were learned in Afghanistan, this concept was designed to reach out for a 1,700yd effective range (not sure if that range, which is about 12 points of damage, will be a good “effective range” benchmark beyond this rifle; we’ll see).

The platform is about 24 lbs, which makes it man-portable by a single (unhappy) soldier. Compare with well over 40 lbs for the M2 even without the 30 lbs of ammo.

It’s an interesting concept, and I’ll have to keep it in mind when I design my Ultra-Tech machineguns!