Based on popular carbine configurations, these XM177E2 Upper Receivers are ready for your next Retro build. The muzzle of the XM177E2 Upper Receiver, is capped by the classic XM177E2 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. Barrels are available with a 1-12 or 1-7 twist. Se Mfg: Brownells Bundles