I vaguely recall that the US experimented with mounting a 75mm autocannon in an airplane, but can’t remember enough to do a search.
I think the A-10 Warthog GAU-8A Avenger 30mm cannon is the largest that actualy got beyond experimental stage into real combat.
This page has video clips of the cannon in use.
From here. From other sites, it seems this gun can fire up to 100 RPM, and was primarily used on a naval mount. That is one big-ass machine gun.
Here - More info the warthog cannon
The gun is 21 feet long and weighs close to 4,000 lbs. The plane is essential a flying cannon.
Well, if you’re asking the question straightforwardly it’s probably the 125mm or even 152mm guns mounted in Russian tanks. They tend to use autoloaders over there for their own reasons.
For aircraft I think you’re thinking about the B-25G or B-25H which mounted a 75mm main gun in the nose. Nasty weapon. I’ve read accounts from pilots regarding the recoil effect on the aircraft. Couldn’t have been fun. Either the navigator or bombadier was a manual loader of that one, though.
I believe This might qualify. I believe the 5" guns used on Aegis cruisers is fully automatic, though I’m still looking for a site the verify this.
There was indeed a B-25 with a 75mm cannon, though as MonkeyMensch said, it needed a loader. The P-39 had a 37mm autocannon firing though the propellor hub.
I’m not sure of the level of automation in the Russian tank weapons (although I think a 152 mm gun would be the biggest, if it qualifies). The U.S. Navy has a number of destroyers armed with the Mark 45 ~ 5-inch, 54-caliber lightweight gun that is fully automated (and approximately 127 mm).
Didn’t the Russkies have a little problem with their early Cold War tank autofeeders stuffing forearms (or heads) into the breech? Or is that just me being jingo Ringo?
Those destroyers (designed in part by yours truly) have a 5" 54 cal and 5" 64 cal guns up forward. These are not my systems, but I think I remember them being capable of 30 RPM. A lot of whupass, but not exactly the spray envisioned for most full auto applications.
Upon review of tomndebb’s link, looks more like 16-20. I think the 5" 64s might have a better rate, though. When I have time, I’ll try to look it up if there is interest.