Once upon a time, a "dissipator" AR was created by taking a 20" rifle-length gas system upper and cutting the barrel down to 16". This allowed the shooter to keep the long sight radius of a rifle for accuracy, but keep the carbine length barrel for maneuverability. The proximity of the gas port to the muzzle, however, played havoc with timing and made reliability a hit or miss proposition. Advances in AR technology have produced reliable carbine and mid-length gas systems, that can be hidden using low-profile gas blocks. This allows for a front sight base to be pinned in the rifle position for the longer sight radius without the worry of the unorthodox gas port location affecting operation. We use the term "mock" to differentiate our dissipator from a cut-down rifle length version. Barrel Length 16" 5.56 Nato Chamber1 in 7" twist Mid-length Gas Phosphate FinishThis upper is sold w/o the BCG & CH. Although we do offer several different options for BCG&CH to add one at check out you would simply select the ones you want to be shipped with your upper. Also when buying them together you will save a little money too.