PPRuNe Pop,
Don't think so (but can't be positive). The British .303 Browning used in WW2 for our fixed wing, turret and free mountings was a derivative of the US .300 AN/M2. It had a rate of fire of 1150 rounds/min (Wiki). IIRC, the Spit loaded 400 rounds per gun (Wiki says 350 for a
Vb), so (theoretically) 20.8 seconds (18.3 for a
Vb) firepower max (as all guns fired simultaneously; there was no provision to select only one or more at a time (as you could do with bombs, say).
But when you had a set of 2 cannon and 4 Brownings, the firing button on your spade-grip was a sort of long "see-saw" button: top was "cannon", bottom was "guns" (or was it the other way round ?) and the middle was "both".
I am no authority, as at OTU (mostly Mk.1s) we only fired a few cannon rounds (on a Mk.II) just to see what it felt like (scary), but no air/air or air/ground gunnery practice. On 20 Sqdn. the guns had been taken out of the XVIs, but I had a fortnight's Gunnery School on Griffon Spits, firing only .303s at flag targets to little effect.
Of course, you would never fire all your 20 (18) seconds in a single burst (and if you did, the barrels would be red hot). All the wartime fighter pilots write about 2-3 second bursts, which I suppose was the maximum you could hold your sights on an enemy who is not going to sit quietly while this is going on (or indeed has seen you coming).
Danny.