Lower Receiver A2 Build KitsLower Receiver Lower Receiver Build Kits Mil Spec. The Lower Receiver A2 Build Kit is a great quality kit that includes all of the necessary parts needed to build out your lower receiver. Features: Mil Spec. Compatibility: Should fit most Mil Spec. AR15 rifle configurations, but refer to your manufacture for more specific compatibility information Common Calibers: . 223, 5. 56, and 300 Blackout Kit Includes: Pistol grip Pistol grip screw & washer