Stainless 12.5" Crusader 5.56mm Barrel W/Switchblock,(US pat #7,856,917). --1 in 7" twist improved polygonal rifling --Carbine length gas system --Extended feed ramps --1/2x28 TPI muzzle threads --Bead blasted finish --Pinned Switchblock with Gas Tube --Medium Contour Weight: 29oz The Switchblock does not fit under a rail. A 7" rail will fit behind the Switchblock. The barrel ships with Switchblock installed and the dowel pin in place. This barrel is designed for our Switchblock that is 1" long(INCLUDED). A standard front sight base is 1.9" long and WILL NOT FIT. Barrel installation should be performed by a qualified gunsmith. Removal and installation of the Switchblock dowel pin and Switchblock is required to complete the assembly process. All NFA Regulations apply. Please check your local and state laws before ordering. Contact your local ATF Field Office with any NFA questions you may have.