What it boils down to is that you will eventually be judged by a "jury of your peers". I'm not an attorney, nor am I considered sworn law enforcement. I am familiar with the justice system and how it works.
For starters, when I'm traveling to and from the range, I make it a point to pick up the necessary refreshments and munchies prior to my trip to the range. If I'm traveling with a buddy and we do stop along the way, one of us will stay with the vehicle. Here in Hawaii, we're not allowed to carry, so shooting at the guy wouldn't be an option. I do have a "high tech" alarm system that has ignition kill, so it will be fairly difficult for some thief to drive off with my vehicle.
Best thing to do would be to secure your firearms at home or other safe place prior to making any side trips.
Suppose I was in the situation for whatever reason and I saw a thug stealing a vehicle full of firearms and ammo, if given a clear shot, I would definitely take it. :)