A link to a series of charts comparing weight of fire and energy :-
http://hipe.uia.ac.be/~gustin/scratch/fgun/fgun-ta.html
The Brownings would be 'synchronised' so that a cone of 0.303 fire converged at 300 yrds (I think)I seem to remember that individual pilots would arrange for this range to be considerably reduced and only open fire at much reduced ranges if possible. There was quite bit of debate as to the desirabillity of using cannon, a number of Aces were able to arrange to fly in the eight gun fighters. Differing wings were built, the type A, retrospectively applied to the original eight Brownings, type B two Hispano 20mm and four .303, the type C which could be fitted with type A or B configurations, or four 20mm Hispanos, also known as the 'Universal' All but about 30 of the spits in the BoB were type A, these being B wing Mk Is. Type E carried two 20mm and two .5 inch but came later.
Regarding the original Spectator post it turns out that there was a Mk VII incendiary Mk 7 bullet, incendiary not explosive, but I think that the comments above still hold. Aircraft could be very difficult to shoot down, I've just come across a page, and promptly lost it, where a US Carrier fighter returns to land on with two 37mm AA hits, six 20mm and 270 rifle caliber bulletholes.!!
Anybody know how many rounds a Spitfire or Hurricane would carry?