Hey Izayo, i think i figured out why the gunshot volume is so loud sometimes.
It depends on the ammo type you're using, for instance, the super penetrator slug will reproduce the sound of the shotgun (louder) based on the amount of zombies you hit with it. Same goes with the Fragmentation slug, but to a lesser extent.
It can be a bother but i don't really mind, i'm leaving it here just in case!