A weapon system is provided. The weapon system includes a receiver and an operating group. The operating group includes a bolt at least partially housed within the receiver; an operating rod (op-rod) assembly arranged to axially translate within the bolt. The operating group also includes a carrier assembly, a bolt assembly, and a recoil assembly. The system further includes a hinge or pivot joint at a connection between the recoil assembly and the carrier assembly, a firing pin lock, an elastomeric cone interface between the bolt assembly and the receiver, a hollowed out piston, and/or tapering on the lug of the bolt assembly.