I was somewhat inspired by both Weiss' weapon, and also kobolds for this one.
The main line of thinking for this one is that the user's semblance would be similar to Weiss' but instead of having the glyphs basically act like spells, for this weapon they act like traps. So for example where Weiss can cast a fireball or ice spell at someone, this creates an invisible glyph somewhere that triggers based on proximity; so an ice shot from this would stick in the ground and wait for the target to walk near to it, then explode in a blast of ice to freeze the target in place. However each trap takes a few seconds to fully set itself, so it's important to lay out these traps ahead of time, or at least kite the enemy long enough for them to take effect.
The main part of the weapon is a normal revolver with 6 powerful shots meant to be used for general combat and self defense. Obviously the fun stuff is in the second chamber where 4 dust canisters are stored instead of bullets. The user can use more or less dust each time a trap is cast, and the more dust used, the more powerful the trap and the wider an area it covers. The types of dust and the traps they make are as follows:
*Fire: Basic damaging trap that causes a large explosion.
*Ice: Generally does little damage but freezes anyone near the trap when it goes off. Generally a smaller area of effect than the other traps unless more dust is used.
*Earth: Creates a large hole to be used as a pitfall trap. This one has some utility to it though as for instance it can be shot at walls or obstacles to create openings.
*Lightning: Largest area of effect with a strong electric burst. Generally non-lethal trap meant for subduing or weakening enemies. Also tends to bypass armor and other defenses.
Each trap takes several seconds to set up, so if shot directly at or near a target it will have no immediate effect. The main advantage of this delivery system is that traps can be set up at virtually any distance provided you can shoot it there. The gun is also relatively quiet when firing traps, which makes it perfect for ambushing Grimm and humans alike.
Also I'm trying to come up with a symbol for this guy because I feel like the gun could use some more details. Like I said this was inspired by kobolds so maybe something related to that? I can't think of a good symbol for stealth and ambushing though...