Just to clarify the different marks of .303 ammunition. PA28 is correct, Mk VII and Mk VIIz were interchangeable, the difference between them was that the former was loaded with cordite as a propellant, the latter with a nitrocellulose-based powder (which is what the "z" suffix means). During WW2, most British-manufactured ammo was Mk VII, the USA supplied Mk VIIz which was issued to the RAF (amongst others). The Mk VIIIz was a special long-range round for the Vickers MMG. It was no more "powerful" than the MkVII, but had a boat-tailed bullet which had superior ballistic performance - giving an effective range of over 4,000 yards. Mk VIIIz can be used in rifles, and in fact some of the surplus ammo available in the UK not long ago was Yugoslavian MkVIIIz. I have never put any through a rifle, but did have the privilege of putting 100 rounds of it through a Vickers in 2002.
As for Spitfire/Hurricane armament - I thought the cyclic rate was nearer 1000 per min, which was not uncommon for an a/c gun; the Vickers K observers' gun had a cyclic rate of 1050. The decision to equip fighters with 8 guns was arrived at based on a mixture of assumptions and simple mathematics. It was estimated in the 1930s that with the speed of modern aircraft a fighter would only be able to hold an enemy aircraft in its sights for a couple of seconds (I forget the exact figure). It had been calculated that it would take a certain number of bullets to cause fatal damage to a bomber. Using these numbers plus the known rate of fire of the gun it was possible to calculate the number of guns that would be required for fighters to shoot down bombers.
OK, I'll put away my anorak now.