Can you shoot a 303 in a 7.65 rifle?

Do not attempt to do this!!!!!!!!!!!!!!

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What he said up there^. With VERY few exceptions, you cannot subsitute one cartridge for another. To do so risks damage to the gun, and injuries to yourself and innocent bystanders. .303 and 7.65 have totally different shapes. The .303 is a rimmed cartridge, 7.65 rimless. Bad idea.