Thats the first person offset, since your in a tank when using that weapon, first person offset really mean nothing, trying moving the marker in the tank turret model.
My real name is a Mammoth, if someone who cares sees this, please change my name to a Mammoth
I use insolence.
Go to insolence, go to the tank turret, open the XML editor, scroll down the the xyz coords, change em. I am reciting from memory, I have only dont this like twice.