The one thing the the 303 Browning did in 1939 was work as did it's ammunition, it took many years to get the .50 cal Browning and various 20mm's to the same level of reliability, as for lasting very long, if I was fighting against A6M's I'd happily take the eight MG's, four with AP four with De Wilde all zeroed at 200m.I know they had some trouble with the cannon, but i also know the cannon worked, which is why basically every nation except the US switched over to 20mm cannon on every fighter. As 'superlative' as the eight (lmg) gun armament was supposed to be, it didn't last too long in the world of fighter design