In d20 modern, the guns have no chnace of misfire. After all, the d20 system does not make you check if your sword breaks from hitting too much or if your bowstring would break. Hence , if you want to adapt the Pathfinder Gunslinger to d20 modern, you would need to modify the Gunslinger Deeds by quite a bit. Here are some of the suggestions I found that has helped me.
But fortunately, it seems that mysterious stranger's replacement deeds seems like a good replacement. As for Gunsmithing, replace it with Gun Training at level 1. Simply replace the weapons with the appropriate gun models and you are good to go.
Would post an example later to see if it is feasible :)