There is the Lyman sabot slug, weighing 525gr. in pure lead and fits in a variety of shot cups.
For my round ball loads, I used only 682" round balls, most likely pure lead.
I got very good results using the gas check off the bottom of spent trap wads WWAA12 white wads mostly, to centre the ball in the barrel. I also used one cup-down on the power charge, with felt and/or card wads in between, to take up the requisite space for a proper folded crimp. I used only AA trap cases for my loads.
In Taofladermause's U-tube testing, their main trouble with various slugs was with getting a poor gas seal, and/or collapsing wad columns.
I didn't have this trouble, as my wad columns were solid wads, hard card under the ball holding cup, then 1/2" to 3/8" fiber wad, then another cup, cup down on the smokeless powder.
With black powder loads, I had one or more hard cards on the powder, then the cup under the ball, and a folded crimp.
My shotgun was made in 1900, or 1898, so was not back-bored and was actually a little tight, at .728".