Catshooter,
Here's what I know and it is all for the 12 gauge. I have loaded single round balls (.69" diameter) and they weigh about 1.2 ounces. A 69 caliber round ball is under barrel diameter in a 12 ga, so I place them in a shot cup and shoot them out of a rifled barrel. Accuracy close to a foster slug.
I have also shot these same 69 caliber round balls out of a 12 muzzleloading smooth bore shotgun. I patched them so them were tight. If I could hit a pie plate 50% of the time @ 25 yards I was lucky. Basically, no spin, no stability, no accuracy, back to the days of the smoothbore musket.
My thoughts are you need a rifled bore to make a single ball fly right.
I think you need some type of gas check to make the thing shoot correctly.
If you are anywhere near bore diameter you will only be able to load one ball. If you want multiple balls you probably have to go to buckshot.
I use WSF winchester powder for my 12 GAUGE sabots & these round balls. 24 to 27 grains, there is a member who goes to 30 grains with 1.2 oz slug, but I found 27 grains of WSF to be plenty hot. Again 12 gauge information that isn't all that helpful for a 20 ga.
Good luck and be safe.
BB
PS: my neighbor has a nasty orange tomcat if you are looking for any targets.
