AR-15 Barrel: 16 M4 RF-85 Treated Contour Barrel with 1:8 Twist Chambered in 5.56/.223 Parkerized Finish Carbine Gas System. - Length: 16 - Profile: Contour - Twist: 1-8 - Finish: Parkerized - Material: 4150 - Caliber: 5.56/2.23 - Gas System: Carbine - Gas Block System: 0.750 - Gas Hole Diameter: 0.068 - Benefits: - The RF-85 process does not add or take away any tolerances therefore you will maintain the accuracy of the barrel. - RF-85 has also been proven to double the life of a barrel opposed to a chrome lined barrel. - It will allow for easy cleanup and give you the durability and longevity you want in a barrel.