Based on the unique early carbine configurations, these GAU-5A Upper Receivers are ready for your next Retro build. The muzzle of the GAU-5A Upper Receiver, is capped by the classic XM177/GAU-5 moderator inspired flash hider, with ports around its radius. This muzzle device is a flash hider, unlike the original XM177 muzzle device, which was technically a suppressor. The flash hider is attached to a lightweight 11.5 or 12.7 inch barrel. Why 12.7 inch? The 12.7 inch model has its muzzle device pinned and welded in place, allowing the overall length of the barrel to reach the required 16" for a rifle. Uppers are available with a 1-12 twist barrel The front sight base is Mfg: Brownells