This is a thought I've had too Blind Ear. My main goal with this was keeping costs low and since I don't have machining equipment, this is something doable by anyone who can buy a tap and a nipple.
I spent $6 for the tap (got a good quality one) and $4 for the nipple. The thread is fine enough it doesn't interfere with the shells chambering, so you can literally unscrew the barrel, unscrew the nipple and go back to regular .22 LR with a little cleaning. You can do it without cleaning, but chamber gets a little cruddy from the BP. Definitely works well enough for small game though.
Hoping to have some updated data soon, but buying a new house and moving, so will probably be later towards Fall.