There were 2 theories of air to air gunnery.
1: Put a huge quantity of bullets out there & prey for enough connects to do damage.
2: Use really big rounds so you only need a very few hits.
The early spits & hurricanes had 8 or 12 browning that could fire 1200 rounds a minute, that's 9,600 or 14,400 rounds combined. each gun had 300 rounds.
The later ones starting with the Mk2B had the 2 X 20mms & 2 X .50 cal, cannon capacity was (IIRC 60 rounds though).
The later ones like the mk9 went all cannon with 4 20mm. they had 120 rounds per gun.
Its all a trade off of performance, weight, size & ammo capacity.
http://www.historyofwar.org/articles/we ... wings.html
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