The Mk 16 was intended to replace the M4A1, the Mk 18 CQBR and the Mk 12 SPR currently in SOCOM service, before SOCOM decided to cancel the order for the Mk 16 Mod 0 (see below).
The SCAR has two receivers: The lower is constructed of polymer, and the upper receiver is one piece and constructed of aluminum [REF].