The MR762A1 uses a proprietary HK short-stroke gas piston in place of the direct-gas-impingement system found on lesser AR15-style rifles that propels exhaust gas into the upper receiver to operate the action. In the "closed" HK piston system, exhaust gasses never enter the receiver, which means the HK runs cooler, cleaner and more reliably. The MR762A1 has a cold hammer forged barrel made from the highest quality cannon-grade steel, producing a barrel that ensures superior accuracy, even after firing thousands of rounds. And while the material and cold hammer forging process themselves greatly enhance durability, HK's processes include many additional steps not taken by other manufacturers to produce superior accuracy and service life, ensuring performance well beyond "mil-spec."- CALIBER 7.62X51MM NATO- MAGAZINE (1) 10rd capacity- BARREL 16.5" medium contour, 1:11" twist, 5/8x24 threaded muzzle- HANDGUARD M-LOK attachment - TRIGGER Two-Stage with 4.5 - 5.6 lb- STOCK 6 Position Adjustable MR