If you're a good shot, a .380 will do fine. Also, using a smaller caliber weapon will do less damage to your car if you're
not a good shot.
I do agree with the whole line of thinking though. In principle, you're spot on!
After some fine upstanding citizen spat out his gum onto my brand new floor mat when the top was down one day when I ran into a grocery store, I no longer leave the top down anywhere out of my line of sight.