The Army began to immediately issue the XM16E1 (re-named M16 on its adoption) to infantry units, and the rifle was initially delivered without adequate cleaning supplies or kit. Moreover, the Army's inability to deliver 5.56mm ammunition meeting quantity and velocity specifications led to a change in powder specification for the 5.56mm cartridge. Unfortunately, the change was made without testing the modified ammunition in the rifle under service conditions. The newly-specified 5.56 ammunition increased the cyclic rate of fire, increasing wear on parts, and the new gunpowder's burning characteristics increased fouling in the M16 rifle.


When the XM16E1 reached Vietnam with U.S. troops in 1966, reports of jamming and malfunctions in combat immediately began to surface. Although the M14 had a chrome-lined barrel and chamber to resist corrosion in combat conditions (a danger learned from WWII Pacific theatre combat experience), the M16/XM16E1 had no chrome-lined bore or chamber.

Several documented accounts of troops killed by enemy fire with jammed rifles broken-down for cleaning eventually brought a Congressional investigation. Later investigations also cast doubt on the veracity of the original 1962 reports of the alleged stopping effectiveness of the 5.56mm bullet, as well as criticism of inadequate penetration (in comparison to the Soviet 7.62mm x 39mm round) when firing at enemy personnel through light cover.