The cool part about shooting black is one nanosecond after you yank back on that trigger, the bullet is, fitting the barrel! Softer alloy and the suddeness of BP results in obturation that takes care of fit. Bullet design is important because what you put into the barrel is going to be different from what comes out of the barrel. It will be shorter and fatter, the design will help control whether or not the change in shape still allows acceptable accuracy.
When I finally heard what Ian and others here were saying about "dynamic fit" and "static fit" with smokeless ammo, and high velocity ammo with cast I realized I had been experiencing these phenomena with BP in my big bore, and why breech seating was so beneficial in my schuetzen rifles.
I really prefer to shoot my BP bullets, "as cast."
^^^ THIS! ^^^
Mind you, my hard-science background is definitely NOT extensive like that of many members here. But HERE GOES anyway. Black powder ignition is a low-order detonation that generally lacks the shattering effects of high explosives and their attendant high-order detonations. I liken its effect on a bullet base to that of a hammer blow or pool cue-to-billiard ball. Smokeless powders DO NOT detonate, per se--they build pressure very quickly (progressive combustion) but it is not an "explosion". I liken the physics of this progressive combustion to a shuffleboard fork.
BP propulsion can and will cause lead or lead/tin cast bullets to upset and fill throats and/or bores/grooves. I am not educated or experienced enough to say conclusively that smokeless powders cannot cause such effects, but I suspect that it is their nature to not prompt same as readily as BP combustion can. When you harden the bullets with antimony, I suspect you lose whatever ability for the alloy to "bump up" almost entirely.
.457" bullets for the 45/70 are almost always a waste of bullet metal. YES, BP can bump them up if the alloy is malleable enough, but if accuracy at distance is your goal you have introduced another variable in the ballistic equation that can and will increase potential error downrange. I share the camp of L Ross, who asserts that "as cast" as close to the critical dimensions present in throat and downbore is a better regimen than trusting to bump-up to take up the slack, Holy Black or Sinful Smokeless.