its a modern foster deer hunting shotgun slug ---( I hunt a lot and know guns and ammo)--- the "rubber" in the back catches the gasses made by the burning gunpowder as they push forward forcing the the outer wall of the slug to expand to fit the barral properly ( they are designed to be used in smooth bore shotguns rather than "rifled" barrel shotguns)-- thus preventing the expanding gasses made by the burning gunpowder from blowing by the slug due to poor fit with the "unrifled" barrel. --- it has it own spin stabilizers on the outside (the ridged part) to keep it steady in flight ---(they are designed for smoothbore shotguns rather than the "rifled" type shotgun ---which are really like a very large bore rifle really )-- thus by using a "foster" slug" itallows a smooth bore shotgun owner to shoot a deer slug without having to get a special barrel to do so (very handy for states where you can shotgun slug hunt only * for deer -- * no rifles or buck shot allowed)--- Ivan