In a PM from Ray the issue of the "revolver cannon" arose again.
I investigated the lack of sound and established that it occurred because the "SND0024.snd" file was missing.
To confirm this I copied the "SND0028.snd" file (30 mm cannon) and pasted it as "SND0024.snd" in the root folder.
When I flew a plane with the revolver cannon I heard the sound when I fired.
Unfortunately I cannot find a "SND0024.snd" file anywhere on my PCs.
Ideally it should be in the "GameData" folder.
So if you have one on your system then please let me know so we can get a copy and solve the problem Edit:
We have found one and it works well