Shooting plainclothes cops that execute a no-knock warrant on your home.
Seriously.
All states--ALL states--have a castle doctrine that allows you to use lethal defense to protect yourself inside your home. A no-knock warrant being executed by cops out of uniform means that you have a reasonable belief that your home is being invaded, and that your life is at immediate risk. Now, admittedly, you probably aren't going to survive that exchange of gunfire. But the state is going to have a really hard time charging you with shooting at/killing a cop if you do.
AR-15 rifles also covers select fire variants. The original AR-15 made by Eugene Stoner was select fire only. The assault rifle/assault weapon distinction is functionally meaningless, and really only applies to the military. Oh, you'll get fudds that will claim otherwise, but they're also the ones claiming that a 1911 is the best gun ever because "TwO WorLD wARs!".