This upper's barrel has a 1:9 RH military twist rate. The barrel is made of 4140 chrome-moly steel, barrel is dimpled for gas block. We recommend using locktite on the gas block screws before final assembly. Our uppers have had extra work put into them. We deburr the feed-ramps and polish the chamber for the highest reliability. We use MIL-SPEC barrels for these. We use nothing but mil-spec uppers, they do have the M-4 ramped barrel and receiver. At a length of 16" it's pretty darn short also it has a 5.56 NATO chamber. We recommend a 62gr bullet for best accuracy.