The SR-25 (Stoner Rifle-25) is a designated marksman rifle and semi-automatic sniper rifle designed by Eugene Stoner and manufactured by Knight's Armament Company. The SR-25 uses a rotating bolt and a direct impingement gas system. It is loosely based on Stoner's AR-10, rebuilt in its original 7.62×51mm NATO caliber. Up to 60% of parts of the SR-25 are interchangeable with the AR-15 and M16—everything but the upper and lower receivers, the hammer, the barrel assembly and the bolt carrier group. SR-25 barrels were originally manufactured by Remington Arms with its 5R (five grooves, right twist) rifling, with twist 1:11.25. The heavy barrel is free-floating, so handguards are attached to the front of the receiver and do not touch the barrel. History In the late 1950s, Eugene Stoner designed the AR-10 battle rifle to equip U.S. troops. It was accurate for an auto-loading rifle, but it lost the competition to the M14 rifle. The patent rights for the AR-10 and the AR-15 were sold to Colt's Manufacturing Company. Colt focused on the AR-15, giving others the ability to capitalize on the AR-10 system. In the early 1990s, Stoner joined Knight's Armament Company. He continued his AR-10 design work and joined it with the direct gas system of the AR-15. The result was the SR-25 (adding together the numbers of the AR-10 and AR-15) which improved the AR-10 design with M16A2 advancements and parts commonality. The original SR-25 was released in the early 1990s and had a heavy free-floating match grade barrel with a fiberglass handguard. It had a flat top upper receiver with a Mil-Std 1913 rail for mounting optics and a 2-stage match grade trigger. The bolt carrier was similar to the AR-10's, being chrome plated and having a captive firing pin retainer pin. The SR-25 was designed specifically to fire open-tip match cartridges. Accuracy was guaranteed at or under 1 minute of angle. At first, AR-10 type 20-round magazines were used, but they were later replaced by steel 20-round magazines resembling those used by the M16. United States Special Operations Command took interest in the SR-25, particularly its high magazine capacity and faster engagement time compared to bolt-action rifles. After some modifications, SOCOM adopted the SR-25 as the Mk 11 MOD 0 in May 2000. Changes included a shorter barrel that could fire M118 and M118LR 7.62×51mm NATO rounds and had a quick detachable sound suppressor mount. An free-floating