Since I'm retired and MSP is at work, I give my best answer.
The frame of the H&R BC is the same as any other H&R SB2 rifle frame, it's just case colored. Depending on year of mfr, H&R will fit any of their barrels to it(except rimfire and the 10ga based barrels) just as they would any other SB2 frame, it isn't limited to just the 45-70 or 38-55 barrel, it comes in the carbine version now with a 45 Long Colt barrel as well, the 38-55 has been discontinued.
Please see barrel fitting in the FAQs and Help sticky, some barrels will fit fine, others may need some fitting. I have forty-five complete H&R single shots, 4 rimfires and 4 shotguns, the rest are centerfire rifles, about half were bought as parts and put together myself, as I said, many fit with no fitting required, some just a bit of work on the latch shelf, others need shimming or cutting at the pivot. Many members buy second hand barrels in the H&R Classifieds, some fit fine, some don't, if the buyer doesn't feel like fitting it themself, they just sell/trade it again, barrel fitting isn't for everyone.
Having multiple frames can make a big difference when it comes to fitting a used barrel, just more options. An option for unmodified barrels(those that haven't been rechambered or otherwise modified) if the barrel has enough underlug left to be cut again, H&R will fit it, that would depend entirely on how it fits before sending it in, if it almost locks up, it's a prime candidate, but if the action closes completely and the barrel is loose, they won't be able to fit it since they grind the pivot to fit, it would need to be done as shown in the FAQs by shimming which they don't do.
Hope this helps,
Tim