Jake, you might want to consider a reduced load (say only 50 grains or so, topped with a cream of wheat or cornmeal filler) for that little carbine. Although Britain never did adopt a "carbine load" in .577 Snider, the recoil was pretty significant with the "shorties" ... as you have discovered! (The heavier 4-band and 2-band rifles aren't so bad ....)
On the other hand, when Britain switched to the .577/.450 Martini-Henry (same bullet weight of 480 grains, but with a powder charge increased to 85 grains) they did not at first adopt a reduced carbine loading for the cartridge .... but found they actually had to delay issuing the lighter carbines for quite some time, until a less punishing carbine load could be issued with them! Recoil with M-H carbines was entirely unacceptable, otherwise ....
That's what I was discussing with my wife. I'm going to make a few rounds and reduce the charge by 5-grain intervals until I find one I like, though I believe that you're right; I'd bet money that 50 grains will be the magic number. Might also add a leather pad over butt-plate, dress it up and make it (the leather pad/stock cover, not the butt-plate) look vaguely Native American. I figure you might have seen that with civilian militia in Canada (I am, of course, no expert, so please tell me if I'm incorrect or way off).
RattlesnakeJack, I can absolutely believe that. I like to think that I'm no wimp in regards to recoil, but I am cringing at the thought of firing that .577/.450 load out of a carbine. Ouch!