I just knocked off the winged sight, and then tapped this one on using a block of wood and rubber mallet. It fit very tightly over the reduced section of the barrel where the orignal site sat, and came to rest upon the transition point on the barrel. It has a set screw on the bottom that cinched it to the barrel to stop it from moving later. The description says it wont fit the 58X series, but it worked great on mine and I have a 581. It adds about 4 inches or so to the overal length of the rifle.