I would measure it. I would/will have nothing to do with a slug-twist double barreled muzzleloading rifle due to the moving projectile (in the other barrel) problem.

I meant to work up loads from 90 2F to 120 2F in 5 gr. increments - in 48" or slower twists.

I can see a 24" shooting accurately with round ball for shot ranges with lighter charges, but 24" is a slug twist and will/should have shallow rifling.