Originally Posted by Sternjaeger II

The vibration you're talking about is absolutely out of scale and wrong.

When you shoot with a machine gun it doesn't rumble or vibrate, it just had one major force vector (which we can call "recoil") that pushes in the opposite direction of the bullet direction. So, Imagining the CoG of the plane as your pivot, the plane would rotate backward on its yaw axis because of recoil, only to be compensated by the other machineguns on the opposite wing and the plane movement vector. As a consequence you can get a flicker on the yaw axis, which varies in its amplitude and frequency according to the guns you're shooting with. The recoils though won't be enough in terms of vector strength or frequency to cause vision blur or flickering like you see in guncameras, but I can tell you that there are other vibrations that can.

I was in a Cessna Caravan which had a prop governor failure, with one of the props going straight into feathering: the vibration and frequency were so intense that the whole world went blurry and your could hear your skull bones rattle! Not a nice experience! It was a second, just the time to switch the engine off, but man the engine could have easily come off its mount!!