Tinker,
The loads I use in the 20 gauge are made up of Lyamn hourglass shaped, hollow based slugs weighing about 375 grains. They were not intended to weigh this heavy but I modified them. I push them out at around 1400 FPS, which is not really fast, but I make them out of pure soft lead. They are loaded and protected by AA shot wads and a material that fills the hollow bases. That keeps the wads from being blown up into the hollow baes and adds more weight to them.
Like muzzleloading bullets made of soft lead, they impart most , if not all, their energy to the animal. The slug and the bore of the gun are both protected by the shotwad. It works just like any other sabot load because the wad imparts the spin to the slug. My barrels are twisted 1 in 20 or 22. With that much rotation, they are very well stabilised and the rotation adds to the killing power, I think. Many people forget that even as a bullet slows down it's forward speed, it retains most of it's rotational movement till it hits something solid. Also, because the wad is catching the rifling, the bullets are not deformed in the loading process.
I am enclosing a picture that shows an unfired slug next to one that passed through four one gallon jugs of water and wrecked the fifth one. It was found on the ground next to the table that held the water jugs. Bob H.