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5.56x45mm NATO is a rifle cartridge developed in the United States and originally chambered in the M16 rifle. Under STANAG 4172, it is a standard cartridge for NATO forces as well as many non-NATO countries. It is derived from, but not identical to, the .223 Remington cartridge.

The 5.56x45mm NATO cartridge with the standard military ball bullet (NATO: SS109; U.S.: M855) will penetrate approximately 15 to 20 inches (38 to 50 cm) into soft tissue in ideal circumstances. As with all spitzer shaped projectiles it is prone to yaw in soft tissue. However, at impact velocities above roughly 2,700 ft/s (820 m/s), it may yaw and then fragment at the cannelure (the crimping groove around the cylinder of the bullet).[citation needed] These fragments can disperse through flesh and bone, inflicting additional internal injuries.[4] Fragmentation, if and when it occurs, imparts much greater damage to human tissue than bullet dimensions and velocities would suggest. This fragmentation effect is highly dependent on velocity, and therefore barrel length: short-barreled carbines generate less muzzle velocity and therefore lose wounding effectiveness at much shorter ranges than longer-barreled rifles.

There has been much criticism of the poor performance of the bullet on target, especially the first-shot kill rate when using firearms that don't achieve the velocity to cause fragmentation.[5] This wounding problem has been cited in incidents beginning in the first Gulf war, Somalia, and ending in the current conflicts in Iraq and Afghanistan. In recent lab testing of M855, it has been shown that the bullets do not fragment reliably or consistently from round-to-round, displaying widely variable performance. In several cases, yawing did not begin until 7"-10" of penetration. This was with all rounds coming from the same manufacturer.[5] This lack of wounding capacity typically becomes an issue at increasingly shorter ranges (beyond 45m when using an M4 or 140m when using an M16 w/ a 20" barrel) or when penetrating heavy clothing, but this problem is compounded in shorter-barreled weapons. The 14.5-inch (37cm) barrel of the U.S. military's M4 carbine generates considerably less initial velocity than its big brother, the 20" barreled M16 and terminal performance can be a particular problem with the M4.