The Mk.IX Spitfire was the first one to carry 0.5 Brownings...The 'Universal' wing had two 20mm gun bays, but only the inboard one was used when the mixed armament was carried. This 20mm weapon was now moved to the outboard bay, and the 0.5 in. Browning was installed in the inboard bay. With the .303 guns deleted, this change in armament meant a change in nomenclature, and the suffix 'E' was added to the Mk. number...only the 'LF and HF' models had the 0.5 in. guns......I feel the combination of 20mm and 0.5 in. machine guns were the optimum armament mixture, the 0.5 in. are virtually mini cannon anyway....anything bigger than rifle calibre has got to be a more 'decisive'-hitting round, especially against bigger aircraft [bombers]; - As in 8x.303 vs. 6x0.5 in., the latter are significantly more destructive....I'm not sure what the rate-of-fire difference is between the 20mm and 0.05 in., but even with less number of guns, I feel this was a reliable, deadly combination....Incidentally, the Spitfire Mk.IX was the first one to have the Gyro gunsight, which really helped where you were putting them 'rounds......