Careful gents, let's get our nomenclature right before we get beat up and the thread deteriorates
I believe the cannon in the P-63 and P-37 was termed the M4 or T9. The M10 is a different animal altogether.
Certainly there was AP ammunition (M80) used in stationary 37mm's but I cannot find any references at all to its use in aircraft. I can find many references that say the ONLY operational round used by the Russians was the M54, which is HE. Did it have some AP functional ability in the P-39? Well, sure. But (my guess) is that was limited to lucky shots on PxKpfw III's or perhaps SdKfz 251's.
As Vinkman pointed out, the P-63 and P-39 were
not tankbusters. The M4 loaded with M54's just doesn't have the stones.
This post is a bit of a bump to keep the Kingcobra dream alive. It's a fun plane to fly in IL-2 Sturmovik and I think it would see some quite a bit of use in AH. Especially the A10 or C5 variant.
For sure, the M4 cannon in the P-39 is an amazing de-acker. Now for the P-63, add 50 mph, factor in 58 rounds (!) of 37mm, maybe 3x500 bombs or 6 HVAR's in a small-wingspan target? Challenge to fly but very effective when done well.