You are good at hitting small targets and precise locations. Whenever the difficulty of the shot is determined by the size of the target, halve the difficulty. You suffer no negative modifiers for stunts that require you to shoot at very small targets; you can shoot a pistol out of the hands of a gunman, or a thrown knife zipping through the air, for example. The last feature allows you to actively defend others as a 1-shot stunt against melee and thrown attacks.
Think of this as a kind of limited telekinesis; if a task can be accomplished with a push or knock in the right direction, you can do the task as a gun stunt. Pushing buttons, switching levers, cutting ropes, cutting that trigger wire that disarms the bomb or sending messages by punching them out with bullets in plaster; all are simple tasks for you. Even if such a task would ordinarily use some other skill (such as the bomb wire, above), you can still use your Guns skill. But tasks requiring complex manipulation (unscrewing the lid on a barrel) is still out of your realm.
You also get to decide how badly you want to hurt someone when you put them down. When you put a mook down, or when a named character fails a DeathTest caused by one of your Guns attacks, you can elect to merely knock them out. They will awaken in fifteen minutes, with att their accumulated Wound points, but otherwise unhurt and not affecte by any of the results of the failed DeathTest.