So... Unfortunately gunners in all the vehicles seem inherit the rotational speed of the vehicle itself (as well as momentum). This can be easily seen in Grizzzmas' video 14 seconds in where the cav brawler is turning left (you can tell from the shadow) and all the tracers veer to the left of the crosshair.
Yes, the bullets should inherit the velocity of the vehicle it is fired from, but NOT the rotation of the vehicle itself (since the gunner's turret itself rotating is compensating for the rotation of the vehicle).
To highlight the problem, at 16 seconds in the vehicle is moving left in relation to the ground the as far as the gunner is concerned so the bullets should be veering off to the left? No. The bullets veer off to the right because the vehicle made a slight right turn.
This makes the experience when gunning for vehicles pretty bad (especially when you're using anything that's non-explosive). I do agree with Grizzzmas here on how if the gunner's turret is aimed at a single spot the bullets shouldn't deviate from side to side due to the rotation of the vehicle (whether or not the driver was pressing A/D).
As a side note, this was way worse for turreted vehicle gunners before patch 3.2 where they :
- Fixed an issue where passenger weapons would no longer be in sync whenever the driver’s turret would rotate, causing bullets to not register when firing at enemies
back then, the turrets rotating would throw off your aim, too (was funny seeing a TOW fly off in a random direction when the tank turret decided to turn)!