The Colt M4 Heavy Barrel features a true 5.56x45mm NATO Chamber for use with both commercial and military surplus ammunition, while the 1 in 7" Twist Rifling is capable of stabilizing light to medium weight projectiles of up to about 77 grains. In 1963 the AR-15 and M16 5.56mm automatic rifles "made Colt the small-arms sensation of modern warfare". Their unmatched accuracy, performance and durability are available in their direct descendent, the Colt semiautomatic-only, sporting rifles. They're ideal for hunting, plinking and competition shooting.