The Vickers 40mm S gun was more powerful, and a lot heavier, about 100 pounds heavier than the M4.
The 37mm M9 was more powerful than either(3000fps), but even heavier(405 lbs vs 320) but still lighter than the 37mm used on the Stukas(650 lbs).
The German Cannon was only slightly more powerful, but carried fewer rounds
Didn't the L-L North Africa P-40s have AT cannon loaded with AP?
This is usually my go-to example of how Mr.Rezun is basically a crazy conspiracy theorist and not a historian, funny that it's been brought up outside such a context.
I did say "if you believe". Though I don't see that the idea of using penals for tail gunners is so completely out there for Stalin's Russia.
The Airacobra's cannon also wasn't particularly effective. Jammed a lot and the muzzle velocity was IIRC pretty low. It also isn't a whole lot of use if you want to knock out tanks with it. I recall an anecdotal story in a book on the Red Air Force where a flight of -39s was called to strafe a column of Panzers and the pilots reported that their guns wouldn't even pierce the top armor.
I remember an anecdotal story from an old Red Air Force fighter pilot who said the reasons that the Soviets loved the P-39 were two-fold:
Its combat performance at low altitudes
Any aircraft you got in front of that gun would be blown out of the skies.
They did okay in Soviet air combat missions (about as good as Yak-1s and LL Spitfires), though. I don't really understand why the Americans disliked them so much.
Because the Americans wanted high altitude fliers for strategic bombing escort, pouncing on low ceiling flying Zeroes (the Aussies and Kiwis hated the P-39 too AIUI), running up kill numbers (which isn't so easy with a P-39), and because the P-39 is an on the deck fighter.
Fighting that low often gives the initiative to the enemy (as it did to the Japanese over New Guinea).
Plus, American fighter pilots seemed to think that their only mission was to shoot down enemy fighters, not shoot down enemy bombers, not protect their own bombers, not do (dangerous) ground support missions where even the greatest of air aces can fall prey to an enemy AA gun with a well-trained crew and good commander.
Reason #40987 why American bomber pilots hated their fighter pilot counterparts, and Reason #1 why the All Black 332nd Fighter Group never had to buy a drink during their deployment in Europe in WWII. Every American bomber squadron in Italy knew full well that the 332nd took their mission of bomber escort more seriously than any of their White counterparts. The 332nd had the lowest kill rate of enemy fighters in Europe for their experience. They also
never lost a bomber to enemy action.
Why didn't the Soviets receive armor piercing ammunition?
No Idea on why they did that
Probably because the Soviets always believed that the best anti-tank weapon was another tank. By the time that P-39s were arriving in large scale numbers, the defensive needs for Soviet AT guns had become less of a critical need than it had been in 1941-42.
So if you are a Soviet Air Force Marshal trying to decide what is the best way to employ the P-39 as an offensive weapon, AP = Wasted Ammunition, while HE = Excellent anti-personnel/soft target weaponry. Kill the soldiers and AT guns while the armor kills the enemy armor.
Just MVHO.