The HK MR556 A4 16.5" 1/2x28 Threaded Barrel CA Compliant Rifle is a semi-automatic 5.56×45mm NATO firearm from Heckler & Koch’s updated A4 series, combining proven piston-driven reliability with practical enhancements for modern use. Built on a gas-operated, short-stroke piston system that helps reduce fouling and improve long-term function, it features a cold-hammer-forged 16.5″ barrel threaded 1/2×28 to accept a variety of muzzle devices. The A4 platform includes a full-length Picatinny rail and modular M-LOK handguard for flexible mounting of optics and accessories, along with ergonomic HK416-style furniture and fully ambidextrous controls for intuitive operation. Designed to meet California compliance standards, this configuration ships with a single 10-round magazine. With its blend of durable construction, adaptable accessory support, and balanced handling characteristics, the MR556 A4 delivers a dependable, versatile rifle platform suitable for range use and practical defensive applications.