First of all, welcome aboard!
Lots of us have bought/traded used barrels, more often than not, they fit fine, some will require some minor gunsmithing to get them to fit. H&R's policy is they will only sell new barrels that they fit to a customer's frame, they won't fit used barrels at all and you can't buy an unfitted barrel from them. H&R barrels aren't one size fits all, so that's why they may need to be "adjusted" to a particular frame.