One of the more persistent myths of World War Two aviation concerns the Russian use of the P-39 Airacobra as a "tank buster," a myth perpetuated until recently . Thanks to James Gebhardt, translator of Attack of the Airacobras: Soviet Aces, American P-39s, and the Air War Against Germany, the actual use of the Bell P-39 by the Soviets can be summarized here.
In short, the Russian pilots flew the Airacobra as "air superiority fighters," and at the low to medium altitudes of air combat on the Eastern Front, they did so with considerable success, against German Fw 190s and Bf 109s. The 216th Fighter Division (later 9th Guards Fighter Division) flew Airacobras from August, 1942 to the end of the war in May, 1945 and counted 28 aces with at least 15 victories.
Anyway the 37mm main gun of the Airacobra was not very efective against armour, his original objetive was to deliver large calibre high explosive shells tracer to ensure the destruction of a formation of bombers that could threat U.S mainland.
Even shooting armour-piercing round the performance was insuficient to penetrate anything more armoured that a Pz I, Pz II ,Sdkfz 251 etc.
Aprox. 20mm of steel plate at 400 metres. Remember that it have a relative short case ( 37x145 rimmed) wich gives a initial speed of 600 m/s.
With no doubt the best gun setup of the entire family of Bell fighters was carried by the P-63D. A elegant bubble-canopy supercharged fighter.
It replaced the old M4 with the M9 wich shot the 37x223SR long bottlenecked case wich improved enormously the penetration on armour.
It was the same cartrigde of the 37mm infantry antitank gun.
Unfortunatly this aircraft not entered in production, his 4x ,50 Browning + 1x37 mm give him a masive firepower that the russians would used with devastating effect againts the new generation of german tanks.