In a situation where the bomber's guns are firing to the rear, won't the rounds fired be losing the forward speed that the bomber is travelling?
Also, I'd think that the fighter would have the overwhelming advantage, in that he is firing fixed guns at a large stable target, while the gunner is firing 2 guns from a flexible position at a much smaller target that is likely maneuvering.
It's hard to say without the actual experience, but if it were my *** on the line, I'd much rather be firing explosive rounds at a bunch of guys standing behind sheetmetal in a schoolbus, than be firing at a single guy in an armored volkswagen with ball ammo.
Someone do the hard math stuff for us, please. The physics involved would seem to be fairly complex.