As some have mentioned, there are several custom mould makers out there (Hoch, Jones, Long, etc.) but they are pricey. Probably run you about $125.00 or so, but they can make just about anything you design. Lee will make something for you also, but be prepared to pay a big set-up fee, plus the cost of the mould if it isn't one they already have a cherry for. Have you tried some of the regular mould makers, Lee, RCBS, Lyman, T/C to see how their moulds would shoot? I have 2 Maxie moulds that cast 406+- gr and 425+-gr. Sounds like you must have a pretty long bullet to weigh that much in a hollow base design. I wouldn't think the bullets would be stabilized being shot from a slow twist barrel as you stated. Emery