They have a plastic ball in the wad that acts on the base of the slug to force out the sides of the slug to engage the barrel. A lot of the old slugs kind of rattled down the barrel, this makes them fit tighter and more consistently.
I have an Ithaca 37 Deerslayer (smoothbore) that with factory open sights gives me 2" groups at 50, and I keep them all in 5-6" at 100 yards. Plenty for where I hunt. Any other basic foster style slug gives me 6-12" groups at 50 yards.
The only slug that comes close to the federal is the traditional Brenneke, and they cost a lot more.
