6 PDR bore is 3.67 inches. With research you should be able find source for cast zinc 6 PDR ball, I have a mould out in the shop. If you are going that big, why not build a real shooter.
The standard for a howitzer chamber is 25% reduction on bore size. 3.62 minus 25% is 2.753 inch for powder chamber diameter. To make the minumum recommend wall thickness you would need a 8.258 diameter barrel. The standard 6 PDR field gun had a barrel 60 inches long.
Remember to meet the standards set down in the law to be exempt as an antique you need to replicate a pre 1899 cannon. Being a muzzleloader by its self is not sufficient to meet the law. Final interpretation of that of course comes from ATF.
Suggest you get your self a set of plans for a specific gun and start looking for piece steel. The two toughest part of build a coon is finding the steel and finding the projectile.
When you are out looking for steel and you find something that looks useful, divide the diameter bot three and that would be the maximum safe diameter bore or powder chamber.
Ask lots of questions,we are more than willing to share or information.
