That's the deal, it really depends on where the BG is in proximity to you when you notice they are armed and figure out it's a robbery attempt as to what the best course of action is going to be. I'm not gonna try and "demolition derby" my way out of there if they are already out of their vehicle and can fill my car window with 6 rounds of 12 ga. or 10-20 carbine rounds as I'm moving past them trying to push their car out of the way.
If they were still inside their vehicle when I noticed the armed people, I think that is a viable option, if not the best option. But in this scenario, they have you blocked in and have already exited their car with the long gun. I'm assuming they are somewhere on the side of your car and not standing directly behind your rear bumper. In this case I would rather open up on them from inside my car, before they can level that long gun at me, or if possible, exit my car to engage where I can move about more.
Again, the bottom line is it all depends on where the BG's are standing when you become aware they have a gun and are robbing you.