I think if you made it a little bit stronger and more percise than the arbalest, but being unable to move while aiming (just implement an animation where you place your gun on one of those staffs), needing a lot of reload time, and most important of all, implement a long delay between clicking and the shot releasing (due to the fact that you need time to put your glowing splint into the hole). And make upkeep expensive. Finally you can give it like 3 slots, ammo needing one slot. If you want a melee weapon you have to use one of those 0 slots weapons.
That way the weapon is really useless in fighting other ranged, but it works well as some kind of "long range artillery" and it is great in fending off cavalry on open maps. Due to the limitation of not being able to move while aiming people could come to the idea to wait until someone comes around the corner to blow off his face, but due to the delay this won't work properly. And you can do the same with a crossbow, anyway.
Seriously, I don't think the weapon would be OP. I can be strong, yes, but most likely not OP. It's just a question of taste if you want it in your mod. But as you already have halberds, flamberges, pikes, side swords, espadas eslavonas, black armour, morions and so on, I think a handgun would not be wrong. I'd like to see it.
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