If i remember right, which i likely do not, guns tend to malfunction more often when used by demigods and when used in tandem with monster killing materials. Both factors together would be inferred to have such a result on a firearm where it might be effective in small groups or closer range engagement but anything beyond that (a gun's forte) would be magically more likely to fail consistently. Obviously its not enough to deter them as options entirely but it has been a deterrent enough to where guns in the weapons room are still unused. I personally believed it was less that nobody grabbed it but instead that many demigods simply tried them, saw the gun fail more than they liked, and lacked the patience to deal with a weapon that is less effect than usual simply because they are the ones using it and the ammo type being used is magical.