A buddy of mine takes the assorted crap from the backstop traps and processes it into cast bullets.
He simply chucks the lot into a suitable bowl and applies heat to it. This produces a lot of smoke as the bits of target and target backing and target frame and linatex burn off so he does this outside. When the smoke has subsided he "waltzes the pot" which is an old plumber's term (he is in fact an old plumber) meaning to gently but thoroughly mix it all. Everything which is not lead floats to the top and can be skimmed off (mostly staples and jackets and ash). Eventually no more crap will float up. Important not to overheat the lead - if the pot continues to produce white powder on the surface then the lead is too hot and the lead is oxidising. He then ladles this into ingots which he later uses to feed his Lee Lead pot for casting. He does not flux the lead (which came as a surprise to me but it works for him). The resulting bullets are visually as good as any cast bullets you can buy, He sizes the bullets and later lubes them with liquid Alox and they work really well. He has observed that the black/grey dust found in the butt traps is in fact mostly fine lead dust so do not sieve it out.