Volume of the main gun is really just economy of the system in the tank. Cheap speakers break up with loud bass.

High pitched MG firing may be the same volume, just sounds louder compared to the bass response of the main. Trust me, I've played bass for nearly 35 years now. For all that time, I've been hauling around piles of gear just to be heard compared to a guitarist. I would have to stereo two 200 watt bass amp heads just to be blown out by a single small 50 watt guitar amp.

If you want a loud sound, use a programmable sound system like a Benedini with an amp, but you'll always be fighting the size constraints of speakers to deliver the bass in the model itself. Without that, you could have the Benedini drive a speaker that would spill you beer when you fired the main gun.