Mage-gun - basically a type of weapon that needs a burst of magical energy to be applied to it in order to fire. The power of the gun is directly related to the power of the spell, thus making a weapon like this deadly when wielded by a magic user and useless in the hands of someone who isn't a mage.

Why does a gun like that exists, you might ask. After all, a magic user can simply cast a fireball or something when in combat, instead of carrying this thing with him. Two reasons. 1) It takes way less energy to fire a gun than to cast a similar spell, and 2) Magic, especailly combat magic, is something you have to LEARN to cast. There are far more people in the world who can USE magic than those who KNOW HOW TO USE it.

Magic in itself is quite expencive, that's why it is being replaced or backed up with technology in everyday life.