What will give or detract from the round ball for accurracy, patched or otherwise, is it's fit in the actual bore, not the case mouth. The Foster slugs will compress enough to go through choke, but dont trust that to happen with a solid ball; you'll need a cylinder bore.
The forcing cone of the chamber will affect this too in the transition from case mouth to bore. The longest case you can chamber, along with the biggest ball for bore, will probably be the ticket. If that wont shoot in you gun better than Fosters then Fosters it is.