I hate to be nitpicky SSGN_Doc because you are huge breath of fresh air around here but the rifling’s don’t impart spin on the slugs. They are there to provide the "squish zone" in the various chokes. The hollow base is three fold. First it provides a means for the skirt to seal to the bore upon ignition of the charge, second it does give for the choke but mostly it gives the foster the nose heavy characteristics it needs so it dosent tumble in flight.....think badminton shuttlecock.
This applies only to factory loads and not Lee or Lyman cast slugs.
Sabot : designed for rifled barrel. Can be shot through a Cylinder (no choke) smooth bore, do not shoot through a chokes barrel of any sort. Reason is the sabot is solid and will not swag down to the size of a choke. Another reason is poor accuracy, it is killing accurate to an extent but the sabot will key hole….or tumble in flight.
Rifled foster slug: foster refers to the hollow base. The hollow base expands from the pressure of the discharge sealing the skirt to the bore. The “rifling” is not for anything other than providing a crumple zone, if you will ,for the slug to fit through various chokes. The rifling does not impart spin on the slug. Designed for smooth bore and can be shot through a rifled barrel with ok accuracy but not as good as a sabot.
Brenneke style:. It is basically an improved foster. The Wad, or shot column stays with the slug through penetration. This offers some advantages in flight stability and extra mass for energy and penetration on the target. This is where you need to pay attention to labeling as some Brenneke brand slugs can be sabots and will be labeled as so.
This is not a Brenneke brand slug but it is a Brenneke style. Take away the wad/column and it is a rifled foster
Take away the wad/column and it is a rifled foster
Here are some of my home cast Lee Key drives .69”
Now someone will come along disagree and call me a know it all but whatever