Good empirical data here (reminiscences by veterans):
http://forum.armyairforces.com/Lared...43-m50398.aspx
Takeaway:
Gunners were mostly trained to deal with fighters attacking via "pursuit curves" (e.g., rear and rear beam attacks), which might account for the greater number of victories claimed by U.S. tailgunners and top gunners.
2% hits vs. a sleeve target towed was considered acceptable air-to-air gunnery standard.
Gunners were trained to shoot no more than 20 round bursts. Guns could be damaged by longer bursts, or "cook-off" could result in unwanted firing which might hit friendly aircraft.
Bullets could be deflected by aircraft slipstream (further reducing accuracy for anything but tail guns fired directly to the rear).