Rim is a bit smaller in diameter (might not extract depending on your gun), case head is a bit smaller in diameter (bigger problem - will bulge on first firing) and it is shorter in length (not that big a deal). I think the 303 is thinner than the 30-40 too. Not a good solution.
Buffalo Arms usually has it in stock - have you tried them?
Remington and Winchester make it once a year - when you find some buy a couple of hundred.
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