Not sure if this is a bug or what. Whenever I use a wirbel in game my trigger stops firing after a while and after monitoring this for a while I have found it is always when the ammo gets down to 1600 - exactly half. I can usually continue by using the bomb button or b key instead, but it's not as easy. If I get a fresh GV or resupply, the trigger continues to work normally.
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Not sure but think I have seen this happen in all guns I have used.