Gawd I hate this....
I can kill buffs with my 'nerf' .50's; but dammit yah gotta be DEAD on target; and concentrate your fire on a single part of the buff. My two favorite targets; the tail and the wing outboard or the engines. (in that order)
If in a decending fast pass; go for his tail... particularly the horizontal stab. Make sure you are not 'hosing' the whole buff.. just concentrate your fire on the tail. What you will do is 'overwhelm' the damage register for this part of the buff and it comes off... like surgery.

If in a climbing attack; go for an outer wing panel. Concentrate the fire on JUST the wing panel. *POP*.. Off it comes....
The hard part... avoiding the buffs return fire while hitting him accuratly. I've had plenty of attacks go sour; with the nose bobbing a bit. Hit sprites ALL OVER the buff; but no apparent damage effects. I would guess the buff is pretty tough; with multiple strikes in the same damage grid to have significant effect. The fuselage seems to be very very tuff.. so I go for the wings and/or tail.
When I'm flying (and gunning from) a buff I try the same thing... I concentrate my fire on the fighters wing. Sometimes; it works. In addition; I fly the B26 like a massive fighter... and most of my kills in it are from those fixed forward firing .50's gunned from the pilots chair. I've had some luck (tho not much) gunnin from the tail, but this seems very ineffective unless you can rudder over some so the top ball or waist joins in. Don't let the fighter sit square on your low six... all you can point at him then is the absurdly weak tail gun. Tromp on them pedals and fox his shot while you bring more turrents into play.
Another note... watch the fire convergence zones in a buff... they do not converge! There seems to be a built in scatter to the train of fire from a buff.. a kinda 'cone of fire' where some pings are landed but never a concentrated stream of fire on the point aimed at. Maybe THIS is why the buffs guns seem so damn feeble.
I ain't saying this is 'right'... it's just what seems to be effective in regards to the way they are used.
Hang