The BC Carbine comes on an SB2 frame, same frame as the 45-70 BC.
The 44mag has come both ways from the factory, 1987-2007 were SB2, in 2008 they switched to SB1 for both the 357 and 44 mags and started fitting acc barrels to SB1 frames, they also dumped the sights in 2008 that were standard on the earlier barrels and made them slick barrels with scope bases only with the exception of the Talo Trapper later.
Why they're still marking SB2(which causes confusion!
) on barrels that come on factory SB1 framed rifles is a big question and one that's been discussed many, many times, best reason I can figure is the barrels are made long before they're fitted to a frame and they just haven't changed the marking procedure.
Tim