I'm sure there are firearms specific training course based around martial arts style. My favorite out of the box martial arts that relates well to firearms is Aikido. Any decent Aikido instructor will know and tell you that every offensive and defensive technic will work open handed or with a sword in your hand (this starts with the very first few things you are taught). The parallel is that in a defensive situation you would want to clear your centerline (not have a sword or gun in the path you are in). Offensively (gun or sword in your hands), it is also similar because your own sword cannot be brought to bear against you if someone grabs your hands; so your own gun during the technique will near be pointing at you (the offensive techniques in Aikido would be impossible if a 2-3 foot sword could be in your hands AND pointed at your body). Aikido isn't my favorite martial arts style but it works well for this application.