The aimer of the weapon points and shoots, using a small handheld device, shaped like a common pistol. Or possibly, like a rifle (with binocular sites).
However, the device wouldn't hold the weaponry, but would instead tell the main weapon where to shoot at.
Inside the handheld part would be a
GPS reciever, a 3d magnetic compass, a 3d accellerometer, and a few gyroscopes.
The GPS reciever would let the device know where it is, the compass and accellerometer would let the device know where "north" and "down" are (i.e., where the device is pointed), and the gyroscopes would correct the accellerometer's reading for short term motions (e.g., if user is using it from a moving vehicle).
In the barrel of the device would be a laser range finder, to tell the distance from the device to the target.
Knowing the device's own location, where it's pointing, and how from from it to the target, the device calculates the GPS location of the target, and sends it to the weapon it's controlling. Pulling the trigger on the device of course causes the weapon being controlled to fire.
The weapon would have the same components for determining it's location and direction, plus an anemometer, plus of course whatever it's firing (a machine gun, mortar, missiles, etc). When given the signal from the handheld device, it would calculate a trajectory to the target and fire.
The big advantage of this design is that the weapon could be a good distance away from the user, and could be unmanned, and firing it would draw attention away from whoever's aiming it.
Also, if the weapon is a machine gun, and is mounted high up (from a tower, or slung below a helicopter), it will shoot downwards towards the target, unlike a normal gun which is often fired from the same height above ground as whomever it's being shot at. This results in any missed shots going into the dirt, instead of resulting in unwanted collateral damage.