Franklin Armory M4-HTF R2 16 inch 5.56 NATO Midlength Barrel Black. 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. Index Pin Installed TDC. Threaded Muzzle Crown. Threaded 1/2-28. Labeled Between Gas Block and Muzzle with Caliber and Twist. Specifications for Franklin Armory M4 5.56 NATO Barrel: Manufacturer: Franklin Armory Color: Black Finish: Salt Bath Nitride Caliber: 5.56x45mm NATO Barrel Profile: M4 Gas System Length: Mid Length Barrel Twist Rate: 1-7 Additional Features: 1/2in-28 Thread .750in Gas Block Package Contents: Franklin Armory M4 5.56 NATO Barrel Franklin Armory 5556: M4-HTF R2 16 inch 5.56 NATO Midlength Barrel Black