In message ,
Ctenos writes
Is there a consistent basis for this designation in aircraft armament?

Not really. "Machine guns" traditionally went up to 12.7mm or 13.2mm,
"cannon" were 20mm and up, and there was sort of a grey area in between:
notably the Soviet 14.5mm and German 15mm weapons.

The Soviets started with an anti-tank rifle and used its ammunition for
a Really Heavy Machine Gun; the Germans made a 15mm weapon, then decided
that it would be better necked out to 20mm and firing bigger rounds with
more payload. (Though they called both MG151/15 and MG151/20 'machine
guns', as well as the 20mm MG/FF, further muddying the waters)

In terms of aircraft armament, it's gone away because 20mm is low-end
for fast-jet armament. Meanwhile, the old rule of thumb that you
couldn't get a useful explosive round in less than 20mm has been
overturned by Raufoss and their 12.7mm multipurpose round.

Basically, pick a position you like and stick to it




--
He thinks too much: such men are dangerous.
Julius Caesar I:2

Paul J. Adam MainBoxatjrwlynch[dot]demon{dot}co(.)uk