The barrels in the M4-1A configuration of the rifle weigh 7.74 pounds -- compared to the M-4's 7.46 pounds -- and absorb more heat. The weight comparisons include the back-up iron sight, forward pistol grip, empty magazine and sling, the Army reported.
Current M4s only fire in single shot or burst mode.
"Soldiers need automatic capability while providing suppression fires during fire and movement," said Command Sgt. Maj. Doug Maddi, Program Executive Office Soldier, Fort Belvoir, Va.
Maddi said soldiers deployed to Afghanistan requested an automatic fire capability. Special Operations Forces have used the M4-1A configuration since 1994.
According to Maddi, the Army's Maneuver Center of Excellence at Fort Benning, Ga., requested the conversion in 2010, which resulted in reliability testing.
The conversion process involves changing out the barrel, adding needed parts and etching out firing mode information on the weapon.
At present, about 300 weapons are being converted per day at Fort Riley in Kansas. Brigade Combat Teams will first be issued the new weapons.
Parts needed for the conversions are being produced by Colt and others.
The total cost for the conversion program, including labor and hardware, is an estimated $120 million.