The Brownells XM177 Flash Hider & Grenade Ring is the appropriate reproduction piece for your Retro carbine build. This flash hider mirrors the original device's exterior dimensions, as well as flash hider cuts and grenade ring. The XM177 Flash Hider fits 1/2-28 threads, is machined from 8620L steel and has a black oxide finish. Flash Hider overall length is 4.335". With the grenade ring, the total length is 4.45". This is the proper muzzle device for the following reproduction builds: XM177 or Colt Model 610 XM177E1 or Colt Model 609 Colt Model 619/620 XM177E2 or Colt Model 629 Colt Model 630 or USAF GAU-5A Colt Model 639 Colt Model 649 or USAF GAU-5A/A