There were few problems with the cannon, themselves; the whole problem was caused by trying to dispose of the empties, after firing. All sorts of curved "deflector plates" were tried, but, due to the wing flexing, none worked. The author of "From Duxford to Karachi" was an armourer on 19 Squadron, and relates the problems they had. Eventually, they gave up on the idea, and, on the Vb (and IIb) the empty case was ejected sideways into a triangular compartment near the wheel well. Once a new feed was perfected, meaning that the guns could, once again, be mounted upright, the problem went away.
The Air Ministry rejected the idea of using 2 x .5" with 2 x 20mm, at first, because they discovered that, from directly behind, a .5" bullet was no better at penetrating German armour than a .303", and from the side, with the average pilot's lack of ability to master deflection shooting, hosing the fuselage with four guns was more likely to disable the pilot. When the gyro gunsight appeared, in early 1944, everything changed, since anyone could master deflection shooting, so the "E" wing was born.
I've been told by a man, who met a man (yes, I know) who'd seen a Spitfire Vc crash; it was one, on test, which had been fitted with 4 cannon, and, when the pilot test-fired them, the wings tore away. Though it sounds somewhat dramatic, it might explain why early "C" wings could not be fitted with 4 cannon, and the 4-cannon wing only became standard in the Spitfire 21.
Edgar