The SR-25 (Stoner Rifle-25) is a semi-automatic sniper rifle designed by Eugene Stoner and made 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 (and therefore on the AR-15/M16), rebuilt in its original 7.62x51mm NATO caliber. Up to 60% of parts of the SR-25 are interchangeable with the AR15/M16 - everything but the receiver, the hammer, the barrel assembly and the carrier/bolt. SR-25 barrels were originally manufactured by Remington Arms with its famous 5R (5 grooves, rounded) rifling, with twist 1:11.25 (1 turn in 11.25" (286 mm)). The heavy 24" (609 mm) barrel is free-floating, so handguards are attached to the front of the receiver and do not touch the barrel.