I don't recollect any problem with the Mag on SB1 frames, H&R will fit the 357 and 44mag to any SB1 frame made since 1987, but some of those that have Max barrels have had some problems with perforated primers, but a switch to different primers would be all thats required to solve that issue, if it's a problem with the Mag, I think the primer switch would be all that's necessary if you handload.
Tim