This video shows some steps using visual markers as aids in teaching a high drive police dog to stop instantly. I am using a whistle in this video, but this can obviously be paired with your voice as well. I am just showing the numerous options of how to implement into your regular program.
I am also using a Thunder 100 retriever launcher (www.thunderequipment.com) to simulate a shotgun sound as gun fire will have a tendency to have a police dog at his highest level of enthusiasm, and this also will give you an opportunity to teach your dog to remain steady until told to go that you will need in exercises or in SWAT work. You obviously do not want the dog attacking everything when it hears gunfire.
If the dog will stay steady with the Thunder 100, which simulates a shotgun sound, he will have no problem with other sorts of gunfire. Having the ability to stop your dog instantly when he is away from you, will keep your legal hassles to a minimum.