British .303 vs 50 Cal M2

Which is why I'd do it my way, I've been shooting my whole life yet I still need a bit of practise to get my eye in if I shoot clays, deflection shooting isn't easy, it's a very hard skill to master.

I'm going to make a guess that the Spitfire's wings were originally designed to hold four .303 machine guns (each), 1930s thinking,
And switching over to Browning M2 .50 and it's ammo (at a later date) would increase the weight of the Spitfire, slowing it down, reducing climb rate and increasing the turn radius, so staying with the .303 would give the Spitfire a better chance of getting it guns on target in the first place.
Later on when the Merlin engine's power was increased and the 20mm cannons became more reliable, it would have then made more sense to upgrade the Spitfire with 20mm cannons.

Clayton Magnet said:

The RAF essentially skipped the step to heavy machine guns, and standardized on cannons as soon as possible. Something that took the AAF and later the USAF about 10 more years to do.

"A Hurricane armed with 4 or 6 M2s would have been formidable in bringing down Heinkels."
Keep in mind that the standard US fighter armament in 1939/40 was one .50 supported by one or two .30's.

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The problem as I understand it was gun reliability; the US version of the 20mm HS was subject to jamming much more than the .50cal was. The English did a better job of building the HS so it didn't jam as much.

I'd hoped to include Shortround's illuminating comments on page 5 in this response but have been unable to make that option work.

Anyway: ref. K14 and other computing sights. They required a smooth touch on "the handlebars" and IIRC 2 seconds or more tracking to yield a solution. For that reason many pilots, including some aces, left them locked in "pegged range." Gabby said that Jerry Johnson was the best shot in the 56th because he'd attended an RAF gunnery course that included a gyro sight in a Spit. "Jerry saw the huge lead that you needed in a full deflection shot."