Is there any documented evidence about the proof firing onto the steel plate? I know the holes of which you speak, I have seen them many times on various carriers, and for years have heard the tale of the proof firing.
As an aside, about a decade back I took a spare front armor section to a machine gun shoot. It was hit about 100 times by various military calibers. The only gun that left an impression was the Boys rifle (holes right through) and a lone hole through from 7.62 X 39 in an area that may have lost it's strength due to the cutting process.
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