WWII 20mm cannon in planes

"Emmanuel Gustin" wrote in message
...
"pendell" wrote in message
om...

... did it have anything to do with the fact that the Germans were
gunning for B-17s and such, and therefore needed a weapon that had a
low rate of fire and less accuracy but a heavy punch?

Like most WWII weapons, the 20 mm cannon were developed
during peacetime, a considerable time before the first B-17s
appeared over Europe. Combat experience did not play a large
role in to the decision. (There was of course some experience in
Spain and China.) It was more a matter of, as engineering usually
is, balancing different factors to find the optimum. The big factors
were destructiveness, hit probability (rate of fire and muzzle
velocity) and weight. The first favours bigger guns, the second
usually favours smaller-calibre weapons, and the third generally
favours bigger guns again (although they are heavier, they give
more hitting power for the same installation weight; for example,
a single .50 is equivalent to about four .30 Brownings.)

The wide consensus during WWII was that the optimum was
around 20mm. Given the same technology, rate of fire and muzzle
velocity were not much lower; the gun was heavier but the
ammunition far more effective. Later several heavy machine guns
were modified to 20 mm cannon (the Soviet ShVAK and B-20, the
German MG 151/20, and the Japanese Ho-5) because they were
judged to be more effective in that form. The USAAF did not follow,
in part because of a different doctrine, and in part because its gun
development budgets between the wars were largely hypothetical
in nature.

Whereas the Americans, whose fighters mostly did escort over
europe, needed a weapon with better accuracy and a higher rof?

It was less a matter of what they needed than what they had. But
the big advantage of the .50 was that a large stock of ammunition
could be carried. A good 20mm cannon would have offered similar
rates of fire and accuracy (although with the limitation that only
four would have been installed instead of six) and more firepower,
but the total available firing time would have been much shorter.
For an escort fighter that was a very important consideration. For
this reason, for example, the USAF decided against a plan to install
four 20mm cannon in the nose of the P-38: The .50s had 500 rounds
(40 seconds of fire) but the cannon only 150 (15 seconds).

--
Emmanuel Gustin
Emmanuel.Gustin -rem@ve- skynet dot be
Flying Guns Page: http://users.skynet.be/Emmanuel.Gustin/





What about planes with multiple fixed guns that had different amounts of
ammunition/fire time -- did the pilot have a selector to determine which
guns would fire or did everything fire when the trigger was pulled and some
guns would run out first?