A little insomnia and a willingness to play with some numbers. These are numbers drawn from the T-55 tank on Wikipedia.
T-55s are 2.4m tall, 3.4m wide, and getting slightly handwavey, coming in at 34 degrees should give you a 50/50 chance of hitting sides or top of the tank.
Shot penetration is measured at “0 degrees”, implying direct hits. So we’re at 56 degrees on top armour, 34 on the flanks. (This excludes marginal gravity-based shell drop, which would make roof hits steeper but flank hits more oblique).
I don’t have a means to hand of easily estimating how many shots would be likely to bounce, but we can say the more oblique the strike, the more likely a shot will. But with enough shots (and the A-10’s cannon is, if anything, good at bringing lots of shots to bear) odds are one will.
If it hits, any oblique shot has to dig through more armour since it’s not going straight through. We’ll get a reduced “effective penetration” as a result. With that said, hits on the sides or rear at our 34 degree dive have a an effective penetration of 89mm on the side/rear armour and 56 mm on the top (based on the 100mm penetration from earlier in this thread).
The T-55 is listed as having 130 to 205mm of armour on the turret (so no penetration),
60mm on the rear of the turret (vs 89mm penetration)
30mm on the roof (vs 56)
79mm upper hull, 20mm lower hull
and 33 to 16mm on the upper hull.
Which is to say any given shell from the gun can punch through a T-55’s armour, with a bit of luck, so long as it hits any part of the tank other than the front armour, or the front or sides of the turret. You don’t need to “tunnel”; with any given hit capable of punching a hole through that tank’s top or flank armour, any individual hit can be a mission kill, and dozens of hits would result in catastrophic damage.
I ran out of patience trying to find stats on the T-72’s armour, especially stats on the thickness of the top of the hull. However, Chechen rebels were able to take these things down with multiple RPG-7 hits, so… maybe the A-10 cannon could do the job, why not? I can only handwave so much.