The GAU-8 was designed to punch thru 76mm armor at 300 meters. [1] The Russian T-90 tank has armor thicker than 500mm. [2]
[1] https://en.wikipedia.org/wiki/GAU-8_Avenger#Specifications
[1] https://en.wikipedia.org/wiki/GAU-8_Avenger#Specifications
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...