Heckler & Koch MR556 A4 Complete Upper Receiver Kit, 5.56mm, 16.5" Barrel, Black - 51000384

This is a complete upper receiver kit for AR-15 platform rifles chambered in 5.56mm NATO/.223 Remington with a 16.5" barrel. The kit features Heckler & Koch's piston and pusher rod system (replacing the standard AR gas tube), M-LOK handguard, threaded muzzle (1/2x28), and includes bolt carrier group, charging handle, buffer weight, and buffer spring. Designed to retrofit Mil-Spec AR-15 lower receivers with improved reliability and reduced malfunctions.