The M15 was developed as a replacement for the Browning Automatic Rifle for use as a squad automatic weapon. It took the basic semi-automatic M14 rifle, added a heavier barrel and stock, a hinged buttplate, a selector switch for fully-automatic fire, and an attached bipod. It fired the standard 7.62 mm NATO (.308 Winchester) round, the same as the M14 and the M60 machine gun.

However, firing tests showed that the M14, when equipped with the selector switch, hinged buttplate and bipod, performed as well as the M15. As a result, the M15 was dropped and the M14 (modified) became the squad automatic weapon. Accuracy and control problems with this variant led to the addition of a pistol-grip stock, a folding metal foregrip and a muzzle stabilizer. The final design was designated as the M14A1.