From The Manufacturer/Wikipedia/People Smarter Than Us:
MasterPiece Arms MPAR 556 Pistol is chambered in 5.56×45 (.223 Rem.) and accepts AR-15/M16 magazines and Drums. The MPAR Sports a heavy stamped-steel receiver,The MPAR 556 employs a short-stroke piston action enclosed by a lightweight, free-floating, two-piece, hard-anodized aluminum handguard. The Gas Piston system delivers the Ultimate in Reliability over the AR15 Gas Impingement system.