The U.S. Army has upgraded a fourth of the 483,000 Colt M4 carbines in its inventory to the newer and more lethal M4A1 model whose new ammunition can penetrate lightly armored fighting vehicles.

The M4 carbine is currently the standard infantry rifle in all branches of the U.S. armed forces. It first gained fame as a weapon used by Special Operations Forces who liked its compact profile and versatility.

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The Army's M4 Carbine Product Improvement Program (PIP) intends to deliver increased sustained rate of fire, durability and enhanced ergonomics. PIP will cost some $120 million over five years beginning 2014. The M4A1 will become the standard U.S. Army rifle in 2019 upon completion of the PIP.

The main PIP enhancements in the M4A1 are a full auto-capability, a heavier barrel and a more consistent trigger-pull. This carbine fires the standard 5.56 × 45 mm NATO round but has been engineered to fire the new M885A1 Enhanced Performance Round (EPR), which can also penetrate body armor and light armored vehicles.

The heavier barrel of the M4A1 better dissipates heat from sustained automatic fire and also helps the carbine fire the M855A1 EPR that has higher proof pressures and puts more strain on barrels. The M4A1 has an effective range of 600 meters.

The weapon's new full-auto trigger group features a more consistent trigger pull, resulting in better accuracy in semi-automatic fire. The M4A1 has an "S-1-F" (safe/semi-automatic/fully automatic) trigger group compared to the M4's "S-1-3" (safe/semi-automatic/3-round burst) trigger group.

The fully automatic capability of the M4A1 is especially valuable in close quarters combat (CQB) and in confined spaces, and is a reason for the weapon's popularity among counter-terrorist units. Upgraded M4A1s have ambidextrous fire control.

The M4A1 is currently used by almost all U.S special operation units including Marine Force Recon, Army Rangers, Army Special Forces, Navy SEALs, United States Air Force Pararescue and Air Force Combat Control Teams. 

TagsColt M4 carbine, M4A1, Product Improvement Program, Enhanced Performance Round