Not necessarily...the M2 on the jeep, for example, fires 1 tracer for every 9 balls (or something like that)...
The most common ammo for WW2 aircraft was the API .50 round, I believe.
Ground troops and jeeps etc, only had to shoot at people. They didn't need to punch through pilot armor ammo bays and fuel tanks.
Don't confuse what's done on the ground with what's done in the air.
P.S. Just because it's not a tracer doesn't make it a ball. There's T, AP, HE, APIT, API, HEIT, all sorts of different rounds used in all those MGs. The MG131, for example, had explosive rounds every X rounds in the belt, as I think did the Italian Safat (maybe?).