AR-15 6mm ARC Barrels Heavy Profile by Faxon. Made from 416R stainless steel with a black nitride finish. Heavy profile with a 1:8 twist (5R) and 5/8-24 muzzle threads. Available in four lengths: 14.5, 16, 18, and 20 inches. Gas system options vary by length: 14.5 inch Carbine, 16 inch Mid-Length, 18 inch Mid-Length, and 20 inch Mid-Length.