Burst Wind Up gives it AP reduction to every weapon equipped as a global bonus, usually abused by swapping to the other weapon slot. Offhand miniguns, knives, LMGs, grenade launchers, fists, etc all get the AP reduction.
The bonus bullets shot per burst functions as normal: only the minigun that fired the shot gets the extra bullets. Though this bug means you can gain even more AP reduction by having two instances of Burst Wind Up from two miniguns.