Rosco Manufacturing M4, Barrel, 556NATO, 16, Mid Length Gas System, Fits AR-15, Nitride Finish, Black BL-16-GVT-556-7-M The 16" M4 5.56 NATO Bloodline Barrel (Mid-Length). This 16" M4 5.56 NATO Mid-Length barrel was designed around the USASOC specs for an Improved Upper Receiver Group with a little bit of the Rosco touch as well. The longer gas system allows for a smoother gas curve which leads to managed dwell times. This combo creates a great balance of speed and performance.