20040116

It isn't the rifle, it's the cartridge

StrategyPage notes that Uncle Sugar is going to give the Heckler und Koch G36 a go (no permalink, look under 14 January 04), under the designation XM8.

Civilians in the less afflicted provinces can sample a functionally identical rifle in the XL8T.

It's all very interesting, Uncle, but the new platform sends the same booolet at the same velocity---or if even shorter barrels than that of the M4 carbine are considered, even less velocity.

Clearly the light and short weapon is needed, for mounted troops, rear-area troops who need to be armed but are not constantly engaged in fighting, and so forth. But there's no need to stay wedded to the weaker cartridge that comes along with today's carbines.

The proposed 6.8mm Remington SPC cartridge is headed in the right direction, using a case that is slightly larger in diameter than the 5.56mm, giving it slightly more propellant capacity, to push a .270" bullet.

This .270" bullet must be short and squat to fit the entire cartridge within the 5.56mm envelope around which modern carbines are built. This short, squat bullet has a poorer ballistic coefficient than it needs to have, so it will lose velocity faster and drop more as it travels.

So a counter-proposal: take the 5.45x39.5mm cartridge case of the AK-74. It has the same case head dimensions of the 7.62x39mm AK-47 round, in fact is is mostly the same case, but the Soviets "Ackleyed" it by forming the case walls straighter, eliminating the case's body taper to increase the propellant capacity.

Just don't neck it down to 5.45mm, stop at 6.5 or so.

The resulting cartridge case will be about 6mm shorter than the 5.56mm original, but have the same or better propellant capacity because it's fatter. Use the shorter case with a longer, higher-ballistic-coefficient bullet, seated out in the case. The resulting cartridge will fit within the 5.56's overall length. Drive a bullet of smaller caliber but higher BC and figure it will retain its velocity better than the 6.8mm.

Instead of 115 grains at 2800 fps, try 140 grains at 2300, assuming the case propellant capacities are identical, and a given mass of propellant delivers the same weight-velocity product whether it's in a longer, narrower space versus a shorter, wider space, which of course it doesn't.

Update: Hornady's Fourth Edition of their Handbook of Cartridge Reloading consists of two volumes, one of load data for various cartridges, and the other of ballistics tables. Lessee: their 6.5mm 140-grain spire point bullet has a ballistic coefficient of .465. When thrown at 2300 fps, it will hold on to 1513 of them at 500 meters. With a rifle zeroed at 200 meters, it will fall almost 71 inches at 500. A 129-grainer (BC=.445) at 2500 fps retains 1633 fps at 500 yards and drops almost 60 inches from a 200-yard zero.

Hornady did not even catalog a 115 grain bullet for .270 in their Fourth Edition handbook. The lightest they go is 130, with a spire point whose BC is .408. Sierra lists a 110-gr spitzer with a BC of .375. With muzzle velocity at 2800 fps, it still has 1717 fps at 500 yards and will drop 76 inches.

I just have a thing for long, narrow bullets with high ballistic coefficients.

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