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So did the Naval Proving Ground just pop over to the Imperial Japanese Navy and say "can we please have a sample of your Yamato Class armor to test"? Obviously not fired from a ship. How was the test done?
Delilah, they most likely have taken spare armor belts designed for the Yamato class off from where it was built like for example threes still armor plating at the the Hamburg dry dock from Bismarck.
It was a gun turret face plate off the 3rd ship that was converted to an aircraft carrier, the IJN Shinano. Captured after the end of WWII and shipped to America for ballistics testing.