The US spent the whole war screwing up the production of .60,.90 and 20mm automatic weapons.
Across the Pacific, the Japanese Army took the 1921 Browning they had a license for, and pumped it up from 7.7mm up to 13mm, then 20mm,25mm and finally arriving at 30mm in late 1944, the Ho-155
30 x 115mm
HE Projectile weight
8.3 ounce/235 g
Velocity 2350 fps/716 m/s
Rate of fire 450rpm
Weight 110 lbs/50 kg
Not as fast firing as some, projectile less powerful than others, but at 50kg, you can mount a few of them, right after WWII.
This is what the USAF should have had in place of the Mk39 when the intended target was large Soviet bombers.
The Mk39 belonged in Navy fighters rather than the Mk12, the last try at making a decent version of the Hispano404 that never worked well.