Interesting thread. Remember seeing a doco on the fate of a WWI tank. The mil-history boffin followed the lead that the Germans would take the projectiles out of their standard issue 8mm Mauser rounds and reverse it.
He went on to prove that such a set would easily penetrate the steel plating as was used on the tank in question. My memory is a bit hazy on the full doco but I think he even went on to test the reversed 190-odd grain FMJ pills on modern steel plate of the same thickness: same result!
Food for thought!