The H&K MR556 (and it's improved variant, MR556A1) is a semi-automatic rifle manufactured by Heckler & Koch. It's chambered for the 5.56 NATO round. Like the HK416, the MR556 uses a proprietary HK short-stroke gas piston in place of the direct-gas-impingement system found on lesser AR-15 rifles that propels exhaust gas into the upper receiver to operate the action. Barrels from M16s, M4s, and other ARs, along with STANAG magazines, are usable sinc the release of 2012-made models.