For something like this, the best person may be yourself. I don't know anyone who specializes in this kind of conversion. You can barrel changing equipment from Midway, Brownells, and other sources. You could send the gun to a smith to do the chambering if you aren't comfortable with doing it. You'd have to standardize a cartridge head to keep from changing the bolt back and forth. Probably an old mauser small ring action would work best. I think that the biggest issue would be getting the short rounds to feed from the magazine. You'd have to have a considerable spacer in the mag, and have the follower cut to fit, and then tinker with spring tension, maybe the feed rails, the extractor, etc.
Rhineland Arms make conversion kits to switch Mauser and Enfields to shoot pistol cartridges. You get a new barrel, an adapter for the mag well, the correct pistol mag, etc. But they aren't switch barrel rifles.
If you can deal with a single shot, what you're looking for already exists on the Handi Rifle, Contender, and Rossi platforms. All of which are a lot cheaper than trying to convert a bolt action or semi auto.
