You are in no way, shape, or form, are you allowed to possess ANYTHING of that nature. Even depleted uranium, is still radioactive, and highly regulated. Even the military supposedly stopped using it in their A-10 Warthogs. In fact, if you should by some chance ever find any, anywhere, chances are, it's a Federal sting, and you'll be arrested, tried, and imprisoned for buying it.
On a side note, if you are aware of a magazine called "The Shotgun News", you will have seen advertisements in the back, that offer the ONE AND ONLY part, that none of the other ads offer, for converting a semi auto, to full auto....those are ATF setups. You get the part, and ONLY that one part, and it's enough to arrest, and try you for possession of an illegal dangerous devise.