Ok, ran some numbers:
Assuming an aircraft weight of 9200 pounds,
270 rounds per gun (so they all run empty at the same time,) 800 rpm per gun,
45 g per bullet, 3000 ft/s.
The total weight of the bullets fired is 160 pounds, and takes 20.25 seconds. The resulting thrust is 740 lbf.
Assuming the aircraft is floating in space with no drag, friction, engine thrust or anything, and ignoring the expanding gases from the powder, the aircraft will recoil 530.6 ft by the end of the 20.25 seconds, and end up moving backwards at 35.7 mph.