Franklin Armory M4-HTF R2 14.5 inch 5.56 NATO Midlength Barrel Black 00-52511-BLK. 5 inch or 16 inch M4 Barrel. 5.56 NATO. 1x7. 4140 Steel. Salt Bath Nitride Chamber Bore and Finish. Carbine or Mid-length Gas System. A4 Barrel Extension 8620 Steel....