Colt M-16A2 5.56mm. Factory 14.5 carbine. The castle nut on the T6 tube, still has the factory peen marks on it, along with the sealer that was put on it by the factory ,Colt. This is a very desirable gun, in this factory original configuration. condition is outstanding. The lower is the registered part, so you can change furniture , uppers, and calibers at your leisure.