500 is right.

However, I have read they were used to pierce the plates, and this I find unlikely.

I have shot much plate in testing various calibers, and speed cuts steel, not big heavy bullets. For example, a 5.45 Russian will commonly dig a deeper hole in plate than will a .375 H&H, as will a 7mmremington Mag. But I have never shot plate with .458 + crowd, except of course for the .50 BMG which is a world unto itself.

Anybody every shot half inch cold rolled plate with a .470?