The Older Foster style Slugs were referred to as "Rifled Slugs" because they had spiral grooves cast into them which would ""Hopefully"" impart some stabilizing spin to them. They were intended for use in smooth bore guns, not rifled tubes. The Brennekes had spiral fins on them & were also designed around smooth bore guns. Either of these early style slugs were also designed to safely pass through a full choke bbl. Not cetain the same can be said of all the more modern ones, but as far as I know ammo is still available loaded with the older style Foster (Rifled) Slugs

Miller/TN
I Didn't Say Everything I Said, Yogi Berra