If you always position yourself strategically, then you might have a few golden seconds before the shit hits the fan. If so, find a way to barricade any possible entryway between the shooter and your location. If it's a door, lock it and/or close it, and if possible, find a large object, such as furniture, to block the shooter's access. After you block the shooter's access to you, find a safe place to seek cover. By doing this, you are making it more difficult for the shooter to make you and your loved ones a target.