The SCAR is manufactured in two main versions; the SCAR-L (Light) and SCAR-H (Heavy). The SCAR-L fires 5.56×45mm NATO, fed from STANAG (M16) magazines. The SCAR-H fires the more powerful 7.62×51mm NATO from proprietary 20-round magazines. Different length barrels are available for close quarters battle and for longer-range engagements. The initial solicitation indicated that the SCAR-H would also be able to be chambered in 7.62×39mm M43 Kalashnikov cartridge and 6.8×43mm Remington SPC cartridge. However, FN is not currently offering them and they likely have been cancelled.

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 Mk 17 will replace the M14 and Mk 11 sniper rifles. However, the weapon will only supplement other weapons while issuing remains at the operator's decision.

The semi-automatic only Mk 20 Mod 0 Sniper Support Rifle (SSR) is based on the SCAR-H. It includes a longer receiver, a beefed-up barrel extension and barrel profile to reduce whip and improve accuracy, and an enhanced modular trigger that can be configured for single-stage or two-stage operation together with either a folding or a non-folding precision stock.[15]

The SCAR has two receivers: The lower is constructed of polymer, and the upper receiver is one piece and constructed of aluminum.[16] The SCAR features an integral, uninterrupted Picatinny rail on the top of the aluminium receiver, two removable side rails and a bottom one that can mount any MIL-STD-1913 compliant accessories. The lower receiver is designed with an M16 compatible pistol grip, flared magazine well, and raised area around the 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.

The SCAR is built at the FN Manufacturing, LLC plant in Columbia, South Carolina, in the United States. Since 2008, FN Herstal has been offering semi-automatic versions of the SCAR rifles for commercial and law enforcement use. These are dubbed the 16S (Light) and 17S (Heavy), and are manufactured in Herstal, Belgium and imported by FN Herstal's U.S. subsidiary FN America of Fredericksburg, Virginia.[17][18] FN America slightly modifies the rifles (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.

The FN SCAR 20S Precision Rifle chambered in 7.62×51mm NATO was introduced in 2019. It is a semi-automatic only civilian version of the FN Mk 20 SSR.[19] In 2020 the FN SCAR 20S 6.5CM variant was announced chambered in 6.5mm Creedmoor.[20][21][22][23] This chambering has been selected by USSOCOM for long-range use.[24]