More thoughts on the 8ga hulls that popped into my head.
I have picked up a few of these shells that were duds and cut them apart to see how they are loaded. The slug has a plastic sleeve around it then two felt wads under and finally a plastic cup wad over the powder. The powder looks like Win 748 or BLC2. Much more "rifle" appearing then "shotgun/pistol" powder, but I don't know that for sure.
I have picked up some of these hulls that are marked "Buckshot", but I have never found one of these as a dud to cut apart.
