The Army began using three-round burst setting in 1986, when it adopted the Marine Corps-developed M16A2 as a replacement for its fleet of M16A1s. The A2 fired the M249 squad automatic weapon’s M855 round and featured a number of modifications over the A1, such as improved sights, a rounded handguard and, of course, three-round burst instead of a full-auto capability.
The Marines developed the burst setting to help riflemen conserve ammunition instead of wasting it during long bursts of full-auto fire. But the Marines and the Army later realized that the mechanics of the three-round burst setting caused an inconsistent trigger pull in the semi-auto mode. This means that the trigger doesn’t feel the same every time a shooter fires, making it harder to shoot with the same degree of accuracy from one shot to the next.
“The trigger is the soldier’s primary interface with the weapon for delivering the round,” said Lt. Col. Tom Henthorn, chief of the Small Arms Branch at Benning’s Soldier Requirements Division.
This is one of the reasons U.S. Special Operations Command equipped its M4A1 carbines with full-auto triggers in the mid-1990s.
The Army’s senior leadership decided to start issuing M4A1s last year as an interim step as it moves ahead with the M4 Product Improvement Program and its improved carbine competition, which could ultimately replace the M4.
“We had some M4A1s on the range … and even the guys from the Army Marksmanship Unit had thought we had [improved] the trigger somehow,” Henthorn said. “The AMU guys were fairly impressed with the trigger.”