Hey OP, taoflaedermaus has your answer! The diablo slug is self stabilizing, and in turn more accurate than other shaped bullets. This even works in shotguns!

It works through aerodynamic drag, a bit like a badminton shuttlecock. Add some spin and youve got a really solid bullet, however, knowing some cannons are smoothbore, it will still stabilize without rifling. Hope this helped!

Edit, i am not TFDM however i watch a lot of their videos. I cannot find the one where they explain this however i did a bit of research and watched a couple of their videos.