Heckler & Koch MR762 A4 Upper Receiver, 308 Winchester/762NATO, 13 Barrel, Threaded 5/8X24, M-LOK Handguard, Anodized Finish, Black, Adjustable Gas Block, Includes Bolt Carrier Group, Buffer Weight, and Buffer Spring 51001919 The latest update from HK-USA for the commercial version of the legendary HK417, the MR762 A4, highlights an improved example of "No Compromise" HK performance. It is now available as a 13-inch standalone upper receiver group.