Considering that it is possible to shoot actual firearms via webcam, a remote-controlled turret shouldn't be too difficult for the technically skilled: a small CCD camera mounted along the barrel of the firearm, a servo to pull the trigger, and a small video panel/control inside the vehicle is enough to get started. A turret wouldn't be that hard to build but it advertises one's vehicle as a potential threat unless one can disguise it as a car-top carrier or something. Disguising and armoring the motors and actuators needed to operate a turret would be another matter--as the driver would have their attention focused on moving the vehicle, a separate gunner's position with weapon controls, video feeds, etcetera, might prove necessary.

The problem with gun ports is that you can't aim, thus some sort of firearm that can be fired in full-auto and "walked" to target is necessary. Gun ports are, though, primarily means for discouraging close approaches than points for precision fire.