Slug guns never had rifled barrels until fairly recently.
I'm sure it varies somewhat with locale but if you went back 10 years in NY, the southern half of which was at the time entirely shotgun only, virtually no one owned a rifled shotgun.
The slugs were rifled, the guns weren't. Even now, unless someone bought a gun fairly recently the odds are good that its a smooth bore.
As an example, my hunting group is 7 guys. Only 2 have rifled barrels. One now, the other guy (me
) got rid of the shotgun altogether.