H&R 1871, Inc. has their own forums. There are several threads about just this subject.
I suggest that you but one of their high powered rifles, like 308, 30-06 or 270, then get the other accessory bbls.
If you do not do it this way you may find that they will not fit some bbls to the receiver that you bought. Something about investment cast vs. forged frames.
Anyway log on to their forums for more complete info.