I tend to agree with Ack-Ack. I love flying buffs, mostly 17's and lancs, and I've experienced both extremes. Some of the time, I lose a drone or lead buff in a single horrible pass, other times I suck up bullets like a sponge.
As for accuracy of buff guns.... I look at three general factors - Patience, Luck, Timing, Luck, Attacking Pilot's position, and Luck...... I find it overall fair. More often than not, I get enough bullets on the attacking plane that they don't' want to continue the attack (oiled, PW, control surface damgae, etc), unfortunately, their friends continue the assault....
