This is the schtick to pick the right load for the job.
You are an expert at tinkering with your guns, selecting ammunition for each target and otherwise using your gear to best effect. As long as you have some time to prepare beforehand and know what type of opposition to expect, you get to add an extra die to the damage of each successful shot.
You are also an expert at making bullets out of unorthodox materials. If you learn what a certain supernatural creature is vulnerable to, you can quickly make bullets out of that material. In many cases, with common substances like silver, you may actually have them in your ammo bag already with a good Guns roll. Such bullets function excaty like normal bullets. Great for bypassing that pesky Damage Immunity.
You have a portable kit so you can make custom bullets in the field. Finding or improvising the appropriate type of bullet during combat is a 3-shot action with a difficulty equal to twice your target's Fortune.
If another character tries to pick up and use one of your modified guns, he will get the bonus as long as he manages to hit with each bullet, but the first miss becomes a jam and negates any further Gunsmith bonuses.