This upper offers a 1:9 RH military twist rate, Chrome lined is also available (ask for details). It is also made of 4140 chrome-moly steel. Our uppers have had extra work put ito 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.