Frank....I'm surprised that some, more in the know, didn't respond to your question. Modern day N-SSA shooters use a projectile about 1/10th of an inch smaller than the bore. I'm sure this varies from team to team. This has been my understanding so far in my quest to shoot my six pounder "live". I haven't checked any of the manuals of the day for their specs. But I really don't remember ever seeing any in my reading. I've got some friends that shoot a twenty pounder Coehern Mortar and they are in the tenth of an inch area. Of course they don't use a sabot like a gun would use. I'm no expert but offer this info from what I have gathered.

Keith