What about a forester slug?
Sure a FOSTER slug,
and a Lee slug will work if the bore and the slug are the proper size, but these are not "mini's". These are more like reverse "tea cups". The cavity is massive, and the weight is in the head of the projectile. So IF you placed say a 12 gauge Foster into an 11 gauge Bess... not going to do well.
High Speed Imagery shows that a Foster slug has a teeny bit of spin, inconsequential, and slugs that are the same style but without exterior grooves, do just as well.
Lee Slugs..............................................................................................................................................Foster Slugs
The Lyman Slug, looks like it would also work, but it's meant to be loaded into a sabot, and into a rifled barrel...
The Brenneke slug many folks think is a Foster slug, but it was invented first, and although it has pronounced ridges, one can see these don't run the length of the slug. The key to the Brenneke styles slug is the wad does not drop away when the slug exits the muzzle. It flies attached to the slug, and acts like the tail of a kite, stabilizing the slug.
Of course they should be tested if one is using a cylinder bore 12 gauge muzzleloader, but in my modern stuff, the Brenneke was vastly better on accuracy.
Again the key to these is a cylinder bore and a slug that closely matches that bore.
LD