.22 to .22mag
This old thread is still often a topic of confusion with theories often more common than real facts. This conversion has been done thousands of times and with a break action gun like my model 24,nothing had to be changed. Even the extractor worked fine with just a little emery paper work. The big concern of most people is the much higher operating pressure of the .22 Mag but it isn't higher. Max pressure levels of both .22LR and .22 Mag are exactly the same at 24000 PSI. Surprised me the heck out of me too. The difference in Rim Dia shouldn't have any effect on misfires.
There is a bore diameter difference of .002" but a bullet swages down in tight bores by the time it travels once its own length. Some have reported less accuracy after the conversion but most don't notice any difference. If the gun isn't accurate to start with, don't expect much improvement after the conversion. The pressure increase due to the smaller bore would much less than expected. 6.72x39 ammo has a .311 bullet but are safe to fire in American made guns with .308 bores. The Ruger Mini 30 is just one example where Ruger has stated it is safe to fire military surplus ammo for the SKS and AK 47 with .311 bullets in the Mini 30 with a .308 bore. I converted my ".22LR over 20Ga model 24" because I often hunted rabbits in open fields including big jacks that would often bolt from too far away for the 20 Ga but frequently would stop running around 100 to 150 yds away and then stand up to look for the danger. Too far for the .22 RF but within reach of the .22 mag with a slight hold over at the longer ranges. The .22 mag is much better for the numerous Coyotes here as well. .22 mag is too expensive for my plinking habits or extended target shooting but inexpensive for hunting and with more than 2X the range and energy of a .22 LR. I wouldn't bother converting a bolt, pump or lever gun. The much longer .22 mag case makes that a very complicated issue. Not a problem with a single shot break action.