I understand, thanks for taking the time.
I see the easy workaround would be to fire from the camera, use a good muzzleflash, animation etc…to “sell” the illusion to the player.
Unfortunately for me, I want a nice trail leaving from the gun and I want the gun to feel deadly accurate without iron sight so what if there is no crosshair on screen but I use a “laser sight” attached to the gun (i.e the socket) ?
This would be equivalent of drawing a (visible) cast from the gun isn’t it ?
It would automatically adjust to the Z and the player will always know where he aim exactly with his gun (that what it looks like in my head right now anyway 
Adding this pic for the sake of clarity (english is not my native tongue)
