You need to go to the company.The barrels need fitted for head space.Read the specifications,only certain years and models will offer another barrel.My 10 gauge for example can only be fitted with another 10 gauge barrel.An SB1 12 gauge can be fitted with a 357 mag or 44 mag depending on year of manufacture but that varies.
I was hoping to pick up a whole rifle. If I could get a used barrel, I'm sure I can have a gunsmith fit it up to a shotgun frame. The company, as far as I can tell won't fit a rifle barrel to a gun that was sold as a shotgun. It's puzzling, really. You would think it would be very simple for them to get into the switch-barrel market, like the T/C's, only much more economical. H&R already sells, or did sell, rifle/shotgun combo's, it would seem a small step from there to selling barrels that would interchange without having to be sent back and fitted. I think the 357 would be a fantastic gun to start mentored youth hunters with. Minimal recoil, but still pushing 158 gr bullets. And I'd have the option of low recoil 38's, 125gr. 38sp +P's, or full power 357 mag loads.