Some years ago a friend of mine was using a fishing sinker mold for his gun, and casting the balls from a NON-FIBERGLASS (I yelled because that's important) automotive body filler. Way cheaper than lead, shot really flat, and even came out black instead of the typical Bondo grey-pink. He used to squeeze the filler into the mold using a cake decorator bag, or something similar. I never saw the process, so I don't know how much time it took.