"Kevin" == Kevin Brooks writes:

Kevin "Ctenos" (reply w/o spam) wrote in
Kevin message
Kevin ...
Is there a consistent basis for this designation in aircraft
armament?

Kevin All cannons *are* guns, so it is OK to refer to them as
Kevin such; but all guns are not cannons. I'd imagine (educated
Kevin guess only) the division between the two comes at the point
Kevin where typical loads can include "payload carrying" rounds,
Kevin which is at the 20mm point (not aware of any *typical*
Kevin loading of HE into rounds under that size, which is why the
Kevin 14.5mm is still refered to as a machine gun and not a
Kevin cannon).

With 'payload' do you mean anything other than 'ball', i.e., HE or
what else? For instance, tracer was available also in 7.7mm rounds I
believe. I think different countries had different names and
conventions for this. Certainly, the Japanese called all calibres up
to 40mm 'machine guns', after the automatic loading.

--
G Hassenpflug * IJN & JMSDF equipment/history fan