I've never had occasion to melt down either a plated bullet or one of the Hornady ENC jacketed bullets with the capped base, so the bursting copper problem hadn't occurred to me. You see, when casting, the shrinkage that occurs at the sprue as the alloy hardens in a mold cavity, so, of course, the reverse occurs when melting. So it makes perfect sense the burst would occur. I expect you can just grab one with pliers and drill a hole in it or hold it against a running band saw blade until it breaks through, but both methods produce sharf or filing scrap from the cutting. I would try just splitting one open with a bolt cutter or else setting them upright on a firm surface, like a block of wood, and hammering a chisel into the nose of each one to split them before heating.
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