Having bought numerous used barrels and having numerous frames, I've never bought one that wouldn't fit, some need a little work on the latch shelf to make em lock up good, but I've only had to fit 2 barrels using epoxy and a shim, the rest have locked up fine. Having multiple frames is a big plus as far as luck in fitting a used barrel without any work. BTW, H&R won't fit a used barrel to your frame, they'll only fit their new barrels from the acc barrel program.
Tim