Here's something else
I think they (Mossberg) make a Moss. 835 smooth bored slug bbl that is actually NOT overbored and is intended for 12ga slugs. The 835 barrels are not "swelled up" but bored out to about 10 ga dia. If I've got it right the barrels intended for slugs are normal 12ga dia. Also seems like two lengths are out there, 18" and the more common 22(24?).
Finally, I recall reading an article about standard Foster rifled slugs many years back, where the author speculated that at extreme range Foster slugs slowed down enough that the flutes actually did impart a degree of rotation and improved accuracy. Way out there, like 300 yds. Which as we know, is not slug range or practical, but this was the guys observations.