It might seem odd to include the 5.56 NATO here, since it is the most popular AR-15 cartridge and the current military chambering, but the round has undergone considerable changes since its adoption. There are essentially two classes of this cartridge. The first and original class features light bullets like the 55-grain found in the M193 round. The second, epitomized by the Mk 262, Mod 0, and Mod 1 rounds developed by Black Hills, uses heavier bullets for long-range, precision shooting.
Built for the U.S. Navy’s Mk 12 SPR and now coveted by snipers and designated marksmen using similar rifles, the Mk 262’s 77-grain Sierra MatchKing projectile has a listed effective range of 600 yards, though it has certainly been put to use at longer ranges. It achieves velocities around 2,750 feet per second (fps) from 18-inch barrels and benefits from advanced propellants. Hornady loads a 75-grain hollowpoint boat-tail bullet in its .223 TAP line with similar specs.
Slower 1:7 twist rates are required to stabilize the heavier/longer bullets. Once they strike soft tissue, the bullets yaw and usually fragment to some degree, greatly increasing lethality over that of the more widely issued rounds like the M855 62-grain Penetrator or Green Tip. Serendipitously, the bigger bullets also perform quite well at close-quarters distances, when fired from short-barreled rifles like the Mk 18 (10.3-inch barrel) fielded by entry teams.
Shooters of all types have benefited from the military’s extensive efforts to improve on the 5.56mm NATO round, and now we have ammo options that extend the range and increase accuracy and lethality.