A second hand barrel may or may not fit, each barrel is individually fitted to the frame at the factory, so they aren't one size fits all, if it doesn't fit, you can fit it yourself using the barrel fitting info in the FAQs, you may also have the option of sending it to H&R for them to fit it for $25 plus the cost to get it there, but there's no guarantee that they'll be able to fit it. If it locks up loose on the frame they won't, but if it won't lock up, there's a good chance they can.
Tim