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If the mouth is 3", I think that means it will be a naval 3"/50 deck gun. In any case (err?), the primer and priming tube have been removed so it "looks" pretty safe.
The 3/50 G indicates it is a cartridge case from a 3 inch 50 gun. The projectile was 3 inch in diameter and the gun barrel was 50 times the diameter long. This guns were on Canadian ships. The Lot CA-500-2 is the ammunition lot number. RP/C indicates that the round had some maintenance done to it at CFAD Rocky Point in B.C.