Looks like cartridge ejection particle effects on certain vehicles (or all of them? Only checked a few) are not taking the vehicles velocity into consideration. Example the casings on the Pawnee:

The reason for that is in class MachineGunCartridge :

moveVelocity[] = {"directionX/2","directionY/2","directionZ/2"};

It would be better like this:

moveVelocity[] = {"directionX/2 + vehicleSpeedLocalX","directionY/2 + vehicleSpeedLocalY","directionZ/2 + vehicleSpeedLocalZ"};

Example:

Now as you can see, the minigun ejections don't woosh away like the flash if you are moving a little bit and looks more like this.