A shotgun shell is solidly filled from the powder to the shot like a rifle case full of COW from powder to bullet. It's the powder surface parallel to and spaced from the bullet's base that acts like a shaped charge and a wall of high pressure meets the bullet's base and acts sideways to form the ring. Unfortunately it is more likely to happen in old barrel steels which may be on valuable rifles. I have not found any need to use a filler in any of my large capacity (relative to the amount of powder being used) cases. If you are using any wad on top of the powder and spaced from the bullet please stop at the first sign of a case resisting extraction. The ring will form where the base of the bullet sits and will progressively get worse.