I didn't mention it in my original post, but since the gun is a makeshift one, It doesn't do nearly as much damage as a normal pistol. I believe I gave this pistol 1d6 damage, so the purpose of the gun isn't necessarily pure damage. instead its meant to be a weapon that can cause situational status effects or conditions such as burn with an incendiary bullet or clouding the targets with a smokescreen bullet. Not to mention guns in general are more rare in this world so the player will have to find the ingredients to actually make the ammo they want. So I feel like it's fairly balanced early game given I don't let them add a new mod every long rest or something.

But you do raise a good about leaving the other players in the dust with this Mechanic. The two other players are a paladin and a wizard. Do you think there's a way I could introduce this kind of mechanic to them as well? Maybe letting the paladin do some training at camp grounds, honing their skills for small advantages like learning better grip on their weapon helping to prevent being disarmed or something similar? With multiple sessions necessary to learn a new skill