Wow, I'm surprised - mine actually sounds a lot like that - and with a very easy fix
What I did was, in
Silent Hunter III/data/Sound replace the file
P02_$MGun_muzzle_flash.wav with a copy of
P19_@AAGun_explosion.wav. It does have a rather similar ring to it I think, when fired multiple times.
There are also custom sound mods for the flak, but often "non-native" sounds from the game cause it to crash, so I avoid them - and this, I thought, is a good way to simulate it.