an embodiment of a method of reconfiguring a firearm receiver system for receiving ammunition from a magazine and from an ammunition belt is disclosed herein . the embodiment of such a method comprises the operations of : providing a substantially as - manufactured oem lower receiver assembly capable of having an ammunition magazine attached thereto for communicating ammunition to an oem upper receiver assembly capable of having ammunition communicated thereto exclusively from the oem lower receiver assembly ; mounting a supplemental upper receiver assembly capable of having belt - fed ammunition communicated thereto on the oem lower receiver assembly ; mounting an ammunition belt feeding assembly on the supplemental upper receiver assembly ; attaching a piston tube assembly to the supplemental upper receiver assembly ; coupling a tappet assembly to the piston tube assembly ; engaging the tappet assembly with a bolt carrier of the supplemental upper receiver assembly ; and attaching an adjustable pressure regulator to the piston tube assembly .