FN SCAR

Overview

Made by FN, the SOF Combat Assault Rifle (SCAR) is a modular weapon system currently in usage by special forces like the U.S. Navy SEALS and U.S. Army Rangers.  FN's design fit the requirements, earning it the SCAR contract and potentially giving it the inside track at replacing the M16A4 for rest of the U.S. military in a future competition.
 
The SCAR is a modular design, with the main barrel and frame of the gun separate so that the former can be replaced without effecting the latter. An extensive rail system is fitted so that a multitude of scopes and optics can be added to the weapon to give the operator's fire greater accuracy, or perhaps enhance visibility in low light conditions. Grips, lights, infrared lasers, range finders, and cameras can also be incorporated onto the extensive rail system. It  features the ability to use the new "swivel" M1203 under slung grenade launcher,  as well as cheek/shoulder adjustable stock. 

Variants

The SCAR comes in 2 main configurations: Mk. 16 and Mk. 17. Mk. 16 was formerly the SCAR-L (Light) firing 5.56x45mm NATO rounds, while the Mk.17 is the former SCAR-H (Heavy) firing the larger 7.62x51mm NATO round. Although virtually identical in nearly every aspect to the Mk. 16 the Mk. 17 is classified as a battle rifle as it fires a full power cartridge instead of the intermediary power cartridge of the Mk. 16. The two versions share a large number of components, and both can be reconfigured easily to use different barrel lengths. The SCAR is also extremely easy to assemble and disassemble thanks to its modularity. Field maintenance ease far surpasses that of the M16A4, and the short-stroke gas piston operating system is considered far more reliable than the direct-gas system used in the M4 Carbine. 
 
SCAR Accuratized[sic] Rifle and infantry Automatic Rifle variants are in testing. Unlike similar derivatives of the AR-15 family, the SCAR specialized variants need fewer unique components and share more with the basic "core" weapon.