Best dogfight gun?

"Tony Williams" wrote in message
m...
The ideal gun for aerial combat will of course combine the best of all
worlds: a high rate of fire, instantly achieved; a high muzzle
velocity to minimise flight time; and projectiles large enough to
inflict serious damage with each hit (requiring a calibre in the 25 -
30 mm range). The optimum weapon among those currently developed may
well be the new GIAT 30M791 revolver, although its weight means that
two GSh 301s (or a GSh-30) could be carried instead, with a higher
rate of fire. If the Russian guns' 30 x 165 ammunition were loaded
with lighter projectiles for a higher muzzle velocity, its aerial
combat capabilities would be improved, at the cost of some loss of
ground attack effectiveness."

Is there more behind the "requiring a calibre in the 25-30 mm range"
than hand-waving to dismiss 20mm guns? Granted, bigger is better,
but why isn't 40mm required or 20mm enough?