This happens to me as well. Stock GWX3, no other mods. I asked a few months back and got no replies. I remember it happening in stock SH3 though, so maybe it's leftover from that.
It's fine at the start of a mission, but the more I use it, the more it skews and skews left/right to the point where my shots aren't even hitting at 700m at the first zoom level. It seems to get worse the more I use the gun within that mission. I have a feeling it may be to do with when the gun gets 'stuck' while you're rotating.
Either way, my solution is just to use it until it breaks, then fire without the zoom. Not useful for long range, but still.
