A little bit of "license" taken with the gun I think. He was firing a .50" calibre machine gun which if it did come from a Spitfire would have been a Colt Browning which was fitted to some later marks. At least one squadron of Mk1b Spits were fitted with the 20mm Hispano canon but were so prone to jamming they were soon withdrawn and the gun not seen again until the Mk5c arrived. The armament fitted depended on the wing type and by far the most Mk1 aircraft were fitted with an A type wing and carried 4 .303 Brownings in each wing. The later "universal" type wing could accept either or a mix of machine guns and canon. Some of these would have been fitted with the gun he was firing. I'm no gun expert but that is my understanding of the general Spitfire development, I'm sure many different "fits" were tried in experimental airframes.
A really interesting programme, thanks for pointing it out. Reminded me of a prize I won as a fourth year apprentice which was to ground run a Mk16E Spit - what a day.
Keith