Why not try the 37 rimmed which is nothing more than the 303 British cartridge necked up to take 375 diameter bullets. Plus there are a bunch of 375 cast bullet molds out there both with and without a gas check. though in the case of the 37 rimmed I opt to use a gas checked bullet.I was going to suggest JES to do the rebore but seeing as how you live in New Zealand maybe they have such a person or shop that specialize in doing rebores. And no messing with the magazine either. Just a suggestion. Frank