I've had 1 or 2 guns cross my bench with this.. it was trigger switch related. The shuttle would not reset properly and float on top of the gun trigger.
SHS trigger contacts would do this on a vfc 416 I was working on. On closer inspection the cutout for the cutoff lever was different shape on the shs shuttle. Put the vfc shuttle back in and it worked.
The other gun was an SRC 416 with a src gearbox with g&p trigger switch and trigger. Once I swapped back to the src bits everything worked fine.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.