I just found the barrel accessory section of the H & R sight and it's almost made my decision for me.

http://www.hr1871.com/Support/accessoryProgram.aspx

Looks like I could buy a simple Handi-rifle or Pardner shot gun (if I understand how this works correctly) and then send in everything but the barrel and get a number of barrels fitted in a what seems like infinite cartridge options.

If it could do .22LR too, I'd be convinced.