Welcome back!
The BC comes on an SB2 frame, same as any Handi made 1987 and later, the Shikari wasn't an SB1 or SB2, it did have a cast iron frame tho, the SB1/SB2 frame designation didn't happen until 1987. Modern 45-70 H&Rs will handle Ruger loads but maybe not forever, it certainly isn't as strong a platform as the Ruger, but lots here use the lower Ruger levels, of course it depends on whos data you use too, Hodgdon and Hornady set the levergun pressure limit at 40kcup which is in or above the Ruger data for everyone else, and FWIW, probably more than anyone needs for that matter, even trapdoor loads up to 28kcup are lethal for pretty much any north american game. If you can stand the recoil, the Handi will work fine as long as you don't give it a steady diet of 50kcup loads, I think that would be detrimental to underlug longevity, more than likely shooter longevity too unless you add some weight and a better recoil pad.
As for the pop opens on the 204, that was likely either oil on the latch/shelf, or you just didn't lock the action closed firm enough, there's a warning on that exact issue in the owner's manual, on a new rifle it's even more critical until the locking parts get use to each other. The only pop opens I've ever had were due to improper cleaning and not closing the action firmly while in a rest, had a couple pop opens today on my 260 Rem Handi, it had oil on the latch, cleaned it and it didn't do it any more, all was good!
Tim
