From what I understand, bigger than .50 cal equals a DD for the firearm. The ammo itself is not regulated, unless you get into things like exploding rounds, like the source of my freinds 'hollow' bullets. They were originally explosive rounds for a 20mm Vulcan. the explosives were emptied out and the detonateor in the nose were cooked off. Same kind of thing for a 40mm grenade launcher or a mortar. AS long as the bullets themselves are not explosive, then only the firearm is a DD. I still don't understand how a 12 ga slug gun bypasses this law, but I'm not complaining!
Is it explosive? You just said yourself if not explosive then not DD. If you mean 12ga. shotguns, it's because they've been exempted as being particularly suited to sporting purposes.