A firearm compatible with pistol magazines and cartridges is disclosed. The firearm may include an upper receiver. The firearm also may include a bolt carrier having at least one fixed bolt carrier key extending therefrom. In addition, the firearm may include a recoil spring guide rod. The at least one bolt carrier key may be slidably attached to the recoil spring guide rod. A recoil spring may be disposed on the recoil spring guide rod between the at least one bolt carrier key and a recoil bumper.