One of the most frequently used Gun cameras of WW II is the Williamson, Air Ministry (AM emblem, only used during the war) G45. These cameras could be placed in the wings of a Spitfier, Hurricane or Typhoon.
It comes in different models (long lens, short lens) and voltages (12V, begin of WWII and 24V, end of WWII and depending of plane.)
This camera was placed in the wings, sometimes replacing a machine gun. The camera was activated by the pilot by pulling the trigger of the firing-system or manually by switching it on.
The camera had different purposes. The footage was not only used as training video or recon video, but also the ACES flew around with these cameras and there the footage is used as prove of confirmed kills.
Advertisements of Williamson camera's
Video (on 1:01 you see a camera mounted in the wing: https://www.youtube.com/watch?v=Zv8rFPLN_Fg
More pictures will come soon!