HM Defense HM15F-556: All HM Defense barrels are match grade, triple honed, and button rifled for superior accuracy. Chambered in 5.56 NATO/.223 Rem, the GuardianF5 4150 Chromoly barrel is 16" in length, features a 1:7 twist ratio, and a mid-length gas system. The custom HM Defense 10 3/8", free float 1913 Picatinny rail is CNC machined from 6061-T6 aircraft-grade aluminum. The rail is compatible with M-Lok mounting system. The GuardianF5 features Mil-spec forged 7075-T6 receiver set that has b been Type III Class II hard-coat black anodized finish. The best part of this rifle is HM Defense's patented HMB Bolt that will not break at the cam pin hole like regular bolts do.