The gun merely lets him lock on to something very precise from any distance, since it uses satellite imagery.
He uses his vector sight ability thing. Firing the gun sends the spell to the satellite, where it bounces back to the target location. He can fire it from his hand if he wants to, but he would nuke himself.
In the first episode you see he fires a regular sniper rifle to the deck of a ship then fires Material Burst from his hand onto the bullet with a pair of binoculars. It fucked up the shore and caused a tsunami, so they said never again.
Like this.