Fairy-Struck
A lot of fairy magic was born of the elements and when fire is used in a particular way—such as igniting all of the bullets in a gun at once—it could make a weapon explode; harming the wielder more than the intended victim.
Non-combustible iron weapons were the way to go with fairies. Something about the chemical composition of the metal reacted to their blood, and if they were actually struck with a piece of the stuff, it would burn their skin. If they were shot with an iron arrow or cut with an iron knife, the iron would poison their blood and without purification, they'd die. So, iron was the metal of choice for extinguisher weapons and when we used it in combination with our psychic abil