Another kind of decoupling is stopping a portion of a circuit from being affected by switching that happens in another portion. Switching in subcircuit A may cause fluctuations in the power supply or other electrical lines, but you do not want subcircuit B, which has nothing to do with that switching, to be affected. A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.
This describes the problem in general but doesn't give an example of specifically where the best place to put them would be. You can put one near the gun, on the servo power lead for each servo, on the power lead for for servo controller, at the relay that switched the gun on, at the power supply for the gun, etc. See the problem? I need someone who understands this stuff to say, "oh, the best place to put the caps would be at _______".
Then there's also the idea Nakarus was talking about:
make yourself a metal sleeve that fits around the area with the motor, and then tie that into ground.
is it best to place it around the gun motor or around the servo motor instead? is this supposed to function like some sort of half-*** faraday cage?
See the 3 different options for caps and diodes that are applied to the motors in the image below?:
Aren't they there to prevent the same problem I'm experiencing? Which configuration would be most effective?