Recon Barrel 5.56mm 16 Inch Stainless Steel With Gas Tube And Pinned Switchblock. One in seven inch twist improved polygonal rifling. Stainless steel, medium contour. Extended feed ramps, bead blasted finish, mid length gas system. 1/2x28 threads. Pinned switchblock with gas tube. Weighs 37 ounces. Switchblock does not fit under a rail. Nine inch rail will fit behind the switchblock. Barrel ships with switchblock installed. Designed for our switchblock that is one inch long (included). Standard front sight base is 1.9 inches long and will not fit. Removal and installation of the switchblock dowel pin and switchblock is required to complete the assembly process. Installation should be performed by a qualified gunsmith.