If a barrel is too thin to tap for screw-in choke tubes or it's chrome-lined (eats reamers/taps), or if the end has been bulged, you can install screw-in tubes. Simply turn the last ¾" of barrel to dimension in chart and solder adapter in place. Mfg: Colonial Arms SPECS: Alloy steel. 2½" (6.3cm) lg. 12 Gauge .867" (22mm) O.D. Accepts Win-Choke-style tubes (Colonial "Invisible" Tubes). Maximum bore diameter .735".