THAT is a section of turret armor plating that was slated to be fitted aboard the IJN Shinano. As the Shinano was converted to an aircraft carrier, the turret armor was never fitted. The test was conducted in October 1946, with a 16/50 Mark 7 gun firing a super heavy AP shell. The tests were conducted at point-blank range. Had the armor been inclined back at the 45 degrees as planned, it would have been impenetrable to guns of any caliber at any range, bar point blank. Read more over at the NavWeaps Technical board.