this screws onto a buff tube, with a 1/2-inch hole size. i used a 1/2 x 36 downsize to 1/2 by 28 thread adaptor with thread protector. push the side through hole that has the thread protector then screw protector on to act as a nut. has to be long enough to be able to thread onto a barrel as well. the cap then screws onto a 1/2 x 28 barrel and a buff tube will screw onto the other end, make some baffles and you've got a winner.