The GAU-8 could not kill T-72s, which entered service during its development cycle.
Everyone fawns over it, but the GAU-8 only had moderate penetrating abilities against T-62s, less against the T-64.
In order to "kill" a T-62, the A-10 had to approach it from very specific directions, at very specific angles of attack. The Soviets knew this so they co-located anti-aircraft artillery with their armor units and any A-10 making a gun run on a tank would have been flying through a hail of anti-aircraft fire.
In order to penetrate a thin sliver of armor on the sides of the T-62, an A-10 pilot HAD to fire from from the side of the tank at "1500 feet away, at 3 degrees of dive, at 320 knots". The Soviets knew this, too.
What direction do you think the barrels of the ZSU-23-4 self-propelled radar-guided anti-aircraft vehicles were pointed, when moving in formation with T-62s?
The PGU-14/B depleted-uranium armor piercing round, the most "anti-tank" bullet the GAU-8 fires, can penetrate 55mm of armor at 1,220 meters and 75mm at 300 meters. There are very few parts of the T-72 that have armor thinner than 75mm.
Also, those penetration figures are for steel. Very rapidly after the introduction of the A-10, the Soviet Union started outfitting all of its main battle tanks with composite armor and additional external armor.
So at best if you pepper a tank with a full magazine of PGU-14/B fired from a GAU-8 you're probably hoping to break its track, poke a hole in an external fuel tank, or puncture its main gun-- not destroy it.
Crews inside modern main battle tanks would probably hardly notice being hit by fire from a GAU-8.
Here is the "A-10 Coloring Book", written by cold war A-10 pilots as a training aid covering how to shoot at T-62. T-62s were only vulnerable from the rear, and thin (hard to hit) slivers along the sides.
The T-64 was harder to kill, and the T-72 almost impossible.
Due to anti-aircraft assets being closely-coupled to Soviet tank formations it was predicted that every single A-10 pilot would be dead two weeks into a hypothetical war with the Soviet Union, and this was 40 years ago.
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.
That's right, the GAU-8 is inadequate for killing modern MBTs, but is complete overkill for any other sort of armored vehicle. It might get lucky and disable a MBT with the autocannon (it may not get through the tanks armor but could still disable the tracks, optics, etc), but really for modern tank killing it would be using something like an AGM-65 Maverick (it can carry up to six.)
Everyone fawns over it, but the GAU-8 only had moderate penetrating abilities against T-62s, less against the T-64.
In order to "kill" a T-62, the A-10 had to approach it from very specific directions, at very specific angles of attack. The Soviets knew this so they co-located anti-aircraft artillery with their armor units and any A-10 making a gun run on a tank would have been flying through a hail of anti-aircraft fire.
In order to penetrate a thin sliver of armor on the sides of the T-62, an A-10 pilot HAD to fire from from the side of the tank at "1500 feet away, at 3 degrees of dive, at 320 knots". The Soviets knew this, too.
What direction do you think the barrels of the ZSU-23-4 self-propelled radar-guided anti-aircraft vehicles were pointed, when moving in formation with T-62s?
The PGU-14/B depleted-uranium armor piercing round, the most "anti-tank" bullet the GAU-8 fires, can penetrate 55mm of armor at 1,220 meters and 75mm at 300 meters. There are very few parts of the T-72 that have armor thinner than 75mm.
Also, those penetration figures are for steel. Very rapidly after the introduction of the A-10, the Soviet Union started outfitting all of its main battle tanks with composite armor and additional external armor.
So at best if you pepper a tank with a full magazine of PGU-14/B fired from a GAU-8 you're probably hoping to break its track, poke a hole in an external fuel tank, or puncture its main gun-- not destroy it.
Crews inside modern main battle tanks would probably hardly notice being hit by fire from a GAU-8.
Here is the "A-10 Coloring Book", written by cold war A-10 pilots as a training aid covering how to shoot at T-62. T-62s were only vulnerable from the rear, and thin (hard to hit) slivers along the sides.
https://imgur.com/gallery/fd4sK
The T-64 was harder to kill, and the T-72 almost impossible.
Due to anti-aircraft assets being closely-coupled to Soviet tank formations it was predicted that every single A-10 pilot would be dead two weeks into a hypothetical war with the Soviet Union, and this was 40 years ago.