I agree with Spanky, I don't "think" you would damage the barrel, but I also suspect that particulair sinker is rather hard for the chore at hand.
FWIW; I have owned a plain-jane SB-2 in 45-70 since about 2000, or 2001. It has become my favorite go-to rifle for about everything from Elk, to Wabbits, and it does it all all with the proper loads....as was so aptly stated, they can go from, "Mild to Wild" in a flash and they will "screw you in the ground" in a New York second.....Trapdoor levels, and just a hair above, makes for pleny gun!
I have never slugged the barrel, although I have all the right stuff, and a bit of experience to do it, and I have never tried to make a .458 Win Mag, or a Quigley out of it.
I have always seen this little rifle as fitting a very unique slot in the world of .45 calibers.
My particular rifle has been a dream to cast for, and I do shoot mostly cast bullets. I have shot everything from round-ball, to heavy 500 plus grainers, and they have all shot well enough for about any hunting situation. I can also get 3 shots touching at 100yds, and I never wonder about the "exact" size of the bore of the rifle that is doing it.
Enjoy that little rifle, that's what they make 'em for.
Respectfully, Russ...