All 39's used the same 37mm m4 cannon.
":Edit: The 37 mm Automatic Gun, M4, known as the T9 during development, was a 37 mm autocannon designed by John Browning[1] and used in the Bell P-39 and P-63 fighters.."
Ever single 37mm m4 used the M54 standard round. Each round had a built in tracer/detonation device.
None could be made invisible, and all* of them blew up after 3 seconds.
only exception was the M80 standard Ap round.
If we had this round (M54) as it was in real life, our 37mm m4 rounds would be very different, we would see "mini puff ack" just as the shells reached max range. "Can you say mobile mini flak?"
What a dang shame, WHAT A DANG SHAME!!