Originally Posted by WALZ
All you need to do is side-tap it with a 1/8 npt-28. You will also have to drill and tap the plug that the pierce pin runs through (8-32 or 10-32 can't remember right now). I use medium locktight to seal set screw. You will eighter need to cut the pierce pin off the valve pin or buy one for a c/a setup. If you are going to mount it on the grip frame you will need to make a solid 3/8 thick spacer to fit tight inside grip frame to mount to or buy a umb for it.
Or for the quick and dirty, just leave a empty 12g in the tube and think of it as a expansion chamber. That way you don't have to tap the face plug or replace your cup seal. It also makes for a easy conversion back to 12g, you just cap the 1/8 fitting and you back to stock class.