I love the P39! We have one here in Niagara Falls at our museum as Bell Areospace made them here. As for the T-9 37mm cannon, it was created by Oldsmobile (no joke) and the Usaaaf called it by M-4, and fired the one seen below, the 145R.
Fired through this nose hub:
This plane really did pack a punch though. As well as the 37mm it had two .50 cals in the nose, and four .30 cals out on the wings!
Too bad Bell Aerospace took out the original super turbocharger.