Steve,
Try the following in this order,

Try civil war projectiles web site at www.civilwarprojectile.com
go to their forum, then to artillery

Also try North-South Skirmish Association at www.n-ssa.org
go to forum index-civil war artillery

Further, try Gray Beards Outdoors (trust me on this one)
Go to forums and scroll down to blackpowder, mortor and cannon forum. It's an extensice site so
continue scrolling down.

These people are knowledgable and serious teckies. Any answer I may attempt would be amateur night at the shooting range.
Good luck
Ron