The SCAR is available in two calibers, and in versions for short and long range combat. It emerged as the winner of a US SOCOM competition to find a new rifle for special forces begun in 2003. Other competitors included entries by Colt and the Robinson Arms XCR. Heckler & Koch had submitted the XM8 as a general issue replacement for the M4 and M16 for the US Army, but the program was canceled.

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 5.56x45mm NATO using improved M16 rifle magazines. The H fires the more powerful 7.62x51mm NATO from a newly designed 20-round magazine (this full-sized cartridge makes the SCAR-H a battle rifle). Different length barrels will be available for close quarters battle and for longer-range engagements. The initial solicitation indicated that the SCAR-H would also be chambered for the 7.62x39mm M43 cartridge. However, FN is not currently offering other calibers.