Great looking rifle.

Round balls would probably prefer a very much slower twist - like 60" or even slower.

There are 12 bore barrels available which have 20" or 22" twists as well.

I don't understand why barrel makers won't make proper twists for shooting the short slugs - perhaps if the demand was better? There is one in the US who makes 36" twists in shotgun barrels but I'm not sure they sell them as blanks. Can't remember the name - been around for quite a spell.