Detachable magazines are kind of cheating. No fixed magazine rifle can compete with a sub-8 lb AR-10 with a 50 round drum. DPMS for example lists their Gen II MOE in .308 Win at 7.25 pounds. Magpul offers their D-50 drum mag. Even the basic Federal/Lake City XM80 149 gr FMJBT produces 2437 foot pounds at the muzzle.
Some adjustments have to be made before we can do calculations. The weight of 50 rounds of 7.62 NATO is going to add 2.75 pounds. The D-50 adds another 1.7 pounds. So loaded, sans optics, the system will weigh around 11.7 lbs. Additionally, the velocity of the XM80 is probably calculated from a 22 inch barrel, such as that found in an M240 or M14. If we lower the velocity to about 2600 fps to account for the DPMS's shorter 16 inch barrel, we get 2237 foot pounds.
So, 50 x 2237 = 111850 total foot pounds of energy / 11.7 = 9559 foot pounds per pound of weapon weight. If we do the calculations without factoring in the weight of the ammo, we end up with 15427 foot pounds per pound of rifle.
And this isn't even messing around finding a truly high powered .308 Win round, like a Hornady Superformance. Nor is it using a build rifle using exotic materials like carbon fiber. If one was willing to shop around and had the dough, they could build a sub-8 pound AR10 with a 20 inch Proof Research carbon fiber barrel and forend that would get more velocity and energy from the rounds while not adding a lot of weight.
