I used to shoot a pile of .575 PURE LEAD OR LEAD PIPE ALLOY round balls through a 20 gauge full choke with homemade sights in a standard federal shotcup with a fiber wad under the ball. Accuracy was pretty good out to about 80-100 yards and then things got a bit iffy. They would put a deer down pretty darn good at close range though. I called em my self contained musket loads. I never saw the need for a 12 gauge slug though, so you are on your own there.
There is quite a few different loads for 12 and 20 roundballs in the Lyman third Edition shotshell handbook if you can lay your paws on a copy.
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