8*303 cals are actually quite lethal, an one second good burst from 250-300 can take down any fighter.
2*303 cals are a pain in the butt tho (109g2 without main gun for example), but 4*303, maybe reinforced with an 50cal, that must be enough.
The problem is, the planes with the small guns are quite slow, the small rounds lose kinetic energy very quickly over d>400, ergo the late war runner simply walk away from you, without taking any visible damage.