The SCAR is manufactured in two main versions; Light (SCAR-L, Mk 16 Mod 0) and Heavy (SCAR-H, Mk 17 Mod 0). The L version fires 5.56×45mm NATO using improved STANAG (M16) magazines. The H fires the more powerful 7.62×51mm NATO from a newly designed 20-round magazine.The SCAR features an integral, uninterrupted Picatinny rail on the top of the aluminum receiver, two removable side rails and a bottom one that can mount any MIL-STD-1913 compliant accessories. It has a polymer lower receiver with an M16 compatible pistol grip, flared magazine well, and raised area around magazine and bolt release buttons. The front sight flips down for unobstructed use of optics and accessories. The rifle uses a 'tappet' type of closed gas system much like the M1 Carbine while the bolt carrier otherwise resembles the Stoner 63 or Heckler & Koch G36. Fabrique Nationale introduced a semi-automatic version of the SCAR modular rifle system, the 16S (Light) and 17S (Heavy), at the end of 2008.[11][12] This version of the SCAR is designed for the law enforcement and commercial markets, and is manufactured in Herstal, Belgium and imported by FNH USA, Fredricksburg, Virginia, United States. FNH USA slightly modifies the rifle (supplying a U.S. made magazine and machining a pin in the magazine well) to be in compliance with U.S. Code before selling them.