I've never used round ball loads in a smokeless firearm but have fired probably a thousand or more in my 20 bore flint fowler. Generally the round ball weighs less than the max shot load. From that, even with the difference in type of fired projectile I would think the round ball load would generate less pressure. Seems the friction of even an unpatched round ball would be less also. Most manufactures of ML smoothbore guns offer data for firing PRB in those same guns.....or they used to.
Keep in mind that my experience shooting round ball loads in smoothbores is entirely with black powder.
Vic
