The slaved gun's firewpower are equivalent to multiple .50s with a single point convergence, like an A-20 except typically the buff formation has more than 6 guns firing at you. And of course, a turret is a much better aiming platform than a A-20.
All of the fighters you've named have convergence issues that reduce their effective range to perhaps half of the turret-mounted .50s. They are pinging you well before you can do anything but waste ammo.
Not really.
The greater physical distance between buff guns from different planes makes convergence an even bigger issue for them. Fighter firepower is much more concentrated at longer ranges.
And aiming is actually easier in a fighter than in a bomber when you follow the golden rule "don't attack from six o clock". When the fighter attacks from high 2 or 3 o clock, the bomber gunner has several angles to track and has a hard time to determine the fighter sspeed and the correct aiming point.
When I'm in American planes with wing mounted 50 cals, I'm usually firing & shredding buffs at D800. One of my favorite buff hunters is the P-51B. "Only" 4 .50cal MGs...
The "secret" to kill buffs is just not to play the bomber pilot's game. Correct application of speed & angles makes buff killing quite easy, only at altitudes above 20k it's getting interesting
Unfortunately, most fighter pilots lack the patience to add another 2-5 mins to get into a favorable attacking position. When I'm flying buffs myself and see a fighter not simply closing slow on my six but climb a bit higher and starting tio overtake me, I know I'm in deep trouble, no matter how his actual "skill level" is.