To be clear, that 500mm number is referring to rolled homogeneous armor (RHA) equivalency. Modern tanks use fancy composite and reactive armor to provide incredible protection. It's not actually 500mm thick, but is apparently equivalent to the protection 500mm of RHA would provide.

These sort of numbers are a bit tricky since some anti-tank weapons are more effective against modern armor than others, meaning the armor will have a different RHA equivalency ratings against different kinds of weapons. For instance, per wikipedia, the M1A1HA's turrent is rated for 600mm / 800mm vs APFSDS (sabot), and 1,300mm vs HEAT.

https://en.wikipedia.org/wiki/Rolled_homogeneous_armour#Curr...

Not directly related, but I recently enjoyed this video comparing the latest models of the USA Abrams & Russian T-90 in a tank battle smackdown.