I discovered this bug when using HAVOC bullet physics. It also appears to be a quite easy fix. The issue is that, with the Gau-19, the bullet spawns inside of the barrel of the gun, causing it to damage itself and blow itself up.

EDIT: Occurs with the LEFH18, Flakzwilling 40/2, ZPU-4, Breda 35, ZU-23-2, PaK 38, Bren LMG, DShK, GAU-19, Kord, M134, MG-15 (both), MG 81Z, RPK.

Also occurs with some helicopters and planes: F-86, AH-1G, Fw 190A-1, Fw 190A-4, Fw 190A-5, Fw 190A-5/U12, Fw 190A-8, Fw 190D-12, Fw 190D-12, Fw 190D-13, Fw 190D-9, Fw 190F-8, Ju 87B-2, Ju 87D-3, Ju 87R-2, Ju-88A-1, and most helicopters/planes that passengers can shoot out of (at certain angles, or at any angle)

It DOES occur with tanks, however tanks don't take MG damages so that doesn't matter.