Ah, yes, but consider this: If the gun is simply a transmitter, then why did Morul stop having his arms possessed the moment he removed the gun?
Perhaps the same reason people fake being captured? To get a better position within the enemy and strike?
Why would his possessed hands try to shoot himself in the chest?
Well obviously because a working head in a possessed body is a liability, since it has control of its own - and perhaps overrides said control.
And why did bandages deanimate the moment the gun was removed?
It can control only one entity at a time?
Why were the infected parts of Morul amputated by his possessor, if those infected parts were actually him?
A tougher one, but perhaps the secondary parts weren't so much amputated (how would he do that anyway) but simply fell off due to accumulated damage? Alternatively...
And how can the possessor control his mind without using mind control?
Well... let's see what it all paints a picture of. A gun is filled with tiny particulate that consumes matter on contact, working like plasma. This gun has the power to control anything in direct contact with it via a single intelligence (as the one in Morul is the same one that knows Faith's possessor), but this control is limited, either in range or by another kind of control present. Morul had succumbed to full possession at about the same time the nanites would be piercing his braincase.
Summation: The gun is a launcher/dispenser for a distributed-intelligence sentient nanite technovirus. When a piece of complex machinery is exposed to it (a human body is just a complex machine in principle), it will penetrate into its structure and attempt to rapidly gain control of everything within its intelligence's communication range. Any sufficiently overtaken piece of machinery capable of transmitting the signal will act as a transceiver, allowing the intelligence to manifest control over any infected machinery in range. The entity performing the hivemind control seems to be limited to one controlled individual, and cannot distribute its attention as a full intelligence, only capable of very primitive control over individual nanoparticles, if it is capable of it at all. It relies on virus-like programming of the nanoparticles, that seek out control centers and pathways, allowing the intelligence to take hold, and ultimately seek new hosts when existing ones sufficiently degrade.
The principle by which the gun works, as well as the presence of a more direct form of control, as with Faith, seems to indicate that the gun is a technological development by a race of body-swapping entities, intended as a transmission method not limited to touch.