In embodiments, a modular rifle includes a lower receiver assembly, an upper receiver assembly, and a coupling mechanism. The upper receiver contains a bolt carrier and a housing enclosing the recoil spring and rod. Within the housing is a counter bore shoulder against which the recoil spring rests. Integrated into the top face of the bolt carrier are compartments, upon which the housing is mounted. Holes in the front of the housing and the upper receiver assembly accommodate the rod. Containing the recoil spring and rod within in the housing mounted to the bolt carrier allows the modular rifle to have a folding, collapsible, or otherwise non-fixed stock. Attached to the rod is a retaining tab, the depression of which allows for the release of the upper receiver from the lower receiver for disassembly of the rifle.