This upper offers a M4 profile barrel with a 1/7 RH military twist rate. The barrel is 4150 chrome-moly steel with chrome lining. We are using Aero Precision USA M4 feed ramped uppers. 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. The hand guards are heat shielded Mil-Spec. BOLT CARRIER GROUP AND CHARGING HANDLE ARE NOT INCLUDED!!!