The fighting rifle trusted by U.S. troops today is the M4A1 carbine, and at the core of this light, compact rifle is a battle-proven barrel design made right here in the USA by FN America. Now, you can get your own barrel made by the same factory and enhance your stock AR-15 or build your perfect M4 clone. This is the BB M4 barrel from FN America. After button-rifling, each FN AR-15 BB M4 barrel gets a chrome-lined bore and chamber that prevents corrosion and also minimizes heat erosion. This helps to extend the life of your rifle, as well as enhance reliability. After chrome-lining, each barrel is high-pressure tested and MPI-inspected to ensure it meets all military standards. Outside, the barrels are treated with a manganese-phosphate finish that resists corrosion. These FN AR-15 BB M4 barrels are all chambered in 5.56 NATO and are available in the following barrel and gas-system lengths: 14.7-inch with a government profile and a carbine gas system, and 16-inch with a government profile and carbine gas system.