A firearm comprises an upper receiver body and a lower receiver assembly.
The upper receiver body is configured for receiving belt-fed ammunition
and the lower receiver assembly is attached to the upper receiver body.
The lower receiver assembly includes a receiver body having an accessory
mounting structure, a trigger group mounting structure, one of an
integrally-formed, non-detachable butt-stock mounting structure and an
integrally-formed, non-detachable butt-stock, and one of a hand grip
mounting structure and an integrally-formed, non-detachable hand grip.