most ammo made was for MGs for ground troops which as a side product could be used for plane MGs.
is why the US stayed with 30 n 50cal so long, as we had a abundant supply already, and to make 20mm for planes would have been specialized.

I think you have no idea what you are talking about.
The United States made almost one million rounds of ammo for the German MG151/20 when we were
thinking of copying it.
Do you really think that the British and Japanese, who spent a hugely disproportionate percentage of their budgets on airforces just produced ammo for their aircraft as an after thought? Really?
Have you ever heard of a German or Japanese fighter, late in the war when they had been decimated and pulverized, not being able to take off with loaded cannons? Crap fuel, yes, but not once have I heard of less than full loads of ammo.
I think you overestimate how hard it is to produce basic 20mm rounds like the Hispano used. German MINE shells, sure, but their basic HE, no.
Well, ok. On place only have I heard of ammo being an issue for cannons, and there is no indication it wouldn't have been an issue for .50 cals had any of the aircraft there used them. One of the reasons given for taking two of the four 20mm cannons off of the Spitfires sent to Malta was to save ammo. But that is about as extreme a case as you can get. It was, at the time, very difficult to supply to the point that these Spitfires were being launched off of carriers hundreds of miles from Malta. The extra pair of cannons were seen as spares.