How much will blackpowder bump up the diameter of the round?
I have an old BP double rifle that I slugged to .512, and have a mold that I had made for it accordingly. Recently, I found another mold for a .50 minnie (same bullet weight), and was wondering if the load would bump up to the full .512? I have been paper patching (both loads actually), and after drying, my diameter for the minnie is at .508. I haven't made it back to the range to try any loads yet, but was just hoping someone would have some experience prior, and let me know if I am setting myself up for some wild flyers!?

If it is a minie, of soft lead, it sure will bump up .004, and more than that if needed. Even a plain based soft bullet will bump up .004 (that is only .002 per side) with black powder. Back in the period, many bullets were smaller than the groove diameters, and all paper patched bullets were smaller.
Of course, for best accuracy, .001 over is best, for me. But from shooting lots of Minies since 1969, they do actually work. If you try smokeless powder, it won't.
I have used Teflon tape, plumbers type, to patch bullets and that would easily make your .508s, into .512s and then the alloy won't matter.

BSS, in Louisiana, will make a swaging die using their version of the old SAS Mity-Mite horizontal press. You send them a few sample bullets that is what you want, and what you want done, and (used to be) about six weeks later you got the press, mounted dies, and the samples you sent.

Rich