according to the video Introduction of P-39 from youtube, the gun mounted on wing side was 20mm or I heard wrong? However the image presented in video truly is close to .5 caliber rather than 20mm cannon.

The video in question [link here] clearly states one 20mm or 37mm along the fuselage centreline, plus two ‘synchronized 50 caliber guns’ [i.e. 0.5in] above the nose and ‘two 30 caliber in each wing’ – at 4 min 15 sec. That matches the standard P-39 armament as published extensively in numerous different sources.

(PS – just a minor correction to nuuumannn’s post, I’d expect US Airacobras would’ve had 0.3in, RAF, and Commonwealth used examples 0.303in.)

Regards,