They did try that in WW1 I think
I think it was on the SE5a to fight balloons (they being higher up than the maximum altitude it could reach). The gun was fixed on a track that tilted it back 90° in order to reload the drum as standard and they did try to fire it straight up in the reload position to get the extra height.
The problem was, the gun jammed a lot when fired that way plus the ammo was incendiary - against the rules to use them in normal dogfights (yes, there were rules!) The pilot needed to be carrying special balloon mission documents in case of capture by the enemy. If they got caught without them, they'd be executed rather than be taken prisoner
By WW2 I think two-seaters were much more powerful and fighters much more suited to shooting bombers (which had better blind spot protection)
(, Sat 2 Dec 2017, 22:21, Reply)
The irony was
the incendiary bullets did practically no damage to the balloons, just whizzing straight through the fabric.
It was the explosive rounds that really lit the torch, so to speak.
(, Sat 2 Dec 2017, 23:10, Reply)