check to see if it has been changed to .22 magnum. this is a common problem , guns were changed to .22 magnum and not marked as such. if a .22 magnum drops right in,this is most likely the case.
Hmmm.... Good idea to check, esp if the marks seem intentional, rather than random.
Sleeving a chamber isn't impossible, just a fiddly way to spend time in the shop, for a fellow that has a lathe and some knowhow. Plenty have been relined with a section of brake Line from the auto parts store. As above, not hardened. Would be damn near impossible to drill ream, rifle, etc., if it were hard.
Liners apparently are available again through Brownell's, as of last year or so, as apparently, the makers of the most commonly used ones, Redman, saw fit to pay up for their Export Registration finally. So i have read. A phone call will sort that out. Ask to speak with the Export desk. The default on the website is that everything cannot be exported, you have to talk to a live person. If you want one by any of the few other sources out there, you have to pretty much rely upon luck or redneck willingness to chance sticking one in the mail (unlikely, as the fines for getting caught are pretty severe). Redman liners are essentially hydraulic tubing that has been button rifled. Results vary, but generally they serve. Not in the same class as the liners made by Parker and such in the days of old, which often shot better than the original bores on many of the target guns of the day.
Making your own liner by turning down a barrel is possible, but a lot of work and fraught with the risk of mucking up the project while it is on the lathe, resulting in looking for another barrel to turn down. Less work to fit a new barrel, if one can be found.
Cheers
Trev