Originally Posted by
POBJOY
Sorley was certainly involved in conducting firing tests on old airframes to assess actual damage 4 v 8 guns. This must have been quite convincing because both Hawkers and Supermarine had to adapt the wing design and gun heating system to suit and only 8 gun versions went into production. Hawkers had little problem with the thick Hurricane wing and ended up with a concentrated group of 4, but Supermarine had far more engineering to do with their 'racer', not only with the fit, but also heating. I suspect the practical trials were the game changer, and we did not have an 'in house' canon at the time. I suspect the Mig 15 large canon fit was more to do with it being able to clobber B29's rather than dog fight with American fighters.
The Spitfire Mk Ib in September 1940 at Duxford was equipped with 2 Hispano 20mm cannon and 4 Browning 303 MG's. These aircraft were rushed into service without adequate trials and as a consequence the cannons were very prone to stoppages. This was partially caused by the large spring that drove the feed mechanism not being capable of overcoming more than a couple of "G". This problem was already know about because the Westland Whirlwind also armed with four nose mounted 20mm Hispanos had the same problem. However the Spitfire had its guns mounted in the wing which increased the effect of "G" and made matters worse.
Incidentally had the Whirlwind not had huge problems with the Peregrine engines and the cannon these aircraft could have made a huge difference in the BoB as a bomber destroyer. But then wars are made up of lots of maybes, etc.
Also later in the war it was established that the most effective 20mm round was not HE but Semi Armour Piercing Incendiary (SAPI). As well as its effect on aircraft SAPI was capable of penetrating the roof armour of the Panzer Mk 4 tanks and made a terrible mess of steam engines.