Grendel LMG

[LRRPF52]

A steel penetrator design based off the 7N6 or 7N10 would facilitate the same velocities as a standard FMJ. Just look at the 5.45x39 velocities for those at 2900 fps out of a 16.3" barrel, with projectile weights at 56gr. M4 with M855 62gr out of 14.5" barrel is 2900+ (not the British down-loaded weak ammo for the L85, but full-power M855). A 120gr 6.5 running quick with the narrow steel rod would smoke some steel. A 130gr LMG load could also be considered, with the right powder. Personally, I wouldn't want crazy penetration in AR's unless dealing with body armor, in which case you would need extreme velocity and light projectiles with small penetrators. Those would have limited effectiveness at distances past 400-500, whereas a standard weight projectile would break through mud, brick, and earthen barriers better out of an LMG. I don't want that for my door-kickers...too much blue on blue.

Either way, you're looking at no less than 4 different projectiles for the dismount force logistics structure: Standard AR load, Standard LMG, AP, Tracer. Notice we haven't even covered DM/Sniper, which is where this cartridge really shines as well, with the 123gr Scenar/SMK types.