Modern gun? 2&3/4" chambers? Some vintage British 12 bore rifles shot very heavy bp loads.
Modern gun, 3" chambers.
Thinking outside the box here (I know, not always a great idea which is why I'm asking questions before doing anything), I used to shoot Sauvestre ammunition from a smooth bore that had delivered great results at 150m. It was a .45 projectile with a high sectional density. If I made a mold similar to that, would anything prevent me from using a subcaliber sabot round with black powder?