"Why Did the M4's 5.56 round require multiple rounds to incapacitate an enemy and what did the Army do about it.

 

This question was on Quora and I snagged it and I added some pictures.  I thought it was a good explanation.

In Iraq, U.S. troops realized enemy fighters could take multiple 5.56mm rounds to the chest and keep shooting. The culprit was just 5.5 missing inches of rifle barrel.

Unlike larger calibers that rely on sheer mass to crush tissue, the 5.56x45mm NATO cartridge relies on velocity. When the standard M855 "green tip" round hits soft tissue at speeds above 2,500 feet per second, the bullet destabilizes, yaws, and violently fragments. This fragmentation creates a massive temporary and permanent wound cavity that causes rapid incapacitation.

The military originally optimized the M855 round for the 20-inch barrel of the M16 rifle, which launched the bullet at nearly 3,100 feet per second. But for urban combat and vehicle mobility, troops needed a shorter weapon. The military transitioned to the M4 Carbine, which has a 14.5-inch barrel.

The shorter barrel drastically reduced the bullet's muzzle velocity. Fired from an M4, the M855 shed speed quickly. At ranges beyond 100 meters, it often struck targets below the 2,500 fps threshold necessary for fragmentation. Additionally, the M855 round was highly "yaw-dependent"—its fragmentation relied on its exact angle of attack upon entering flesh. If it struck straight-on, it could pass through five inches of tissue before tumbling. On thin fighters, the bullet often passed straight through without fragmenting, a phenomenon troops called the "ice-pick" effect. The target was left with a neat puncture wound that, while eventually fatal, did not immediately stop the threat.

The military ultimately fixed this with a metallurgical redesign of the bullet itself, developing the M855A1 Enhanced Performance Round (EPR).

Instead of the M855's lead core and internal steel tip, the M855A1 features a solid copper core beneath a much larger, exposed steel penetrator. This three-piece construction (jacket, penetrator, core) is designed to act almost like a hollow-point upon impact. It aggressively yaws and fragments within the first two inches of tissue, independent of its initial angle of attack. The M855A1 maintains reliable fragmentation at much lower velocities, allowing the shorter-barreled M4 Carbine to inflict consistent terminal damage at nearly double the range of the older ammunition while simultaneously providing better barrier penetration against windshields and cinderblocks.

The M855A1 Enhanced Performance Round features an exposed steel penetrator and a solid copper core. Source: Wikimedia Commons.