This upper has a Rosco Mfg 4150 M4 profile barrel with a 1/9 RH military twist rate QPQ finish. We are using our Flat top M4 feed ramped uppers. At a length of 16" it's pretty darn short also it has a 5.56 NATO chamber. We recommend 55gr to 62gr bullet for best accuracy. The hand guards are single wall heat shielded carbine. Front sight is F-Marked installed with taper pins after barrel finishing. Default flash hider is a A2. Default option uppers will ship in 5-10 business days after order. Any upgrades may add to the build time.