The HK MR762 A4 16" 5/8×24 Threaded Barrel Upper Receiver Kit is a complete upper assembly designed to retrofit or upgrade compatible 7.62×51 mm NATO/.308 Win rifles to Heckler & Koch’s piston-driven A4 platform. This kit includes a cold-hammer-forged 16″ barrel threaded 5/8×24 for attaching muzzle devices or suppressors, a full upper receiver with forward assist and convertible charging handle, a free-floating rail-system handguard, a complete bolt carrier group, proprietary buffer and buffer spring, and an adjustable gas block for tuning performance. The piston and pusher-rod operating system keeps hot gases and carbon out of the receiver area, enhancing reliability and reducing fouling compared with traditional direct-impingement designs. Because it is sold as a parts kit and not a complete firearm, it can be purchased without a Federal Firearms License, though compliance with all applicable federal, state, and local regulations is the buyer’s responsibility.