mickey is right, but depending on the gun, condition and a host of other variables, it may be possible to set the barrel back (by shortening the shank and re-threading a portion of it) and get the flanged chambering from the existing belted chamber. Without knowing what you want to rechamber (and what it's worth) I'd suggest you take it to a gunsmith for evaluation of your options.

In the single-shot this would probably require a new forend or at least some forend work... agian, weigh the value of what you've got against the value of the results you want.