A light-weight, shoulder-firable, armor-penetrating gun characterized by a recoiling barrel confined to coaxial movement by a pair of longitudinally spaced bushings within the gun housing, the recoiling barrel being adapted to carry rearward the elongated bolt carrier containing the bolt for compressing the recoil spring, and a stop mechanism associated with the barrel and the housing for limiting the rearward travel of the barrel for early disengagement between the barrel and the bolt carrier, and a barrel spring in the front of the housing for returning the barrel to its normal battery position.