An improved firearm having a barrel coupled to an upper receiver where the receiver includes a passageway extending generally parallel to the longitudinal axis, and a guidance feature extending along the passageway generally parallel to the longitudinal axis. The firearm also includes a lower receiver coupled to the upper receiver and a recoil spring assembly comprising a mating attachment structure and a disassembly button disposed on a lateral side of the upper receiver. The mating attachment structure is attached to an attachment structure of the lower receiver to attach the lower receiver to the upper receiver. Actuation of the disassembly button detaches the mating attachment structure from the attachment structure to detach the lower receiver from the upper receiver.