You probably already own a superb defensive weapon. It’s your car or truck. People just don’t want to hurt their vehicles, they don’t think about intentionally damaging them. This is such a widely-held belief that nobody would expect you to use your car to defend yourself. Use of a vehicle to cause harm to others is sometimes called vehicular manslaughter in court. I prefer to think of it as using an available resource in a prudent manner to protect yourself.

Energy is measured in foot-pounds. Dirty Harry’s big 44 Magnum generates 740 foot-pounds of muzzle energy (Winchester 240 grain soft point). My wife’s truck generates 7,000 foot-pounds at 5 miles an hour, 26,740 at 10 miles an hour and 106,970 at 20 miles an hour. Those are all speeds that could be reached in a parking lot. This is a lot of self-defense.

Since you’re awake and alert, you watch what’s happening around your car so you can get into it safely. Once inside, get it running. First thing, don’t make calls or shuffle papers. If you’re approached on any side, you have the option of shifting into drive or reverse and cutting your car toward the threat. Just wipe the threat up against a nearby car, pole, whatever’s available. You are holding a fantastic advantage because you’ll be doing something completely unexpected. Most cars won’t stop small arms fire from the sides or rear, so your reaction time is critical. From the front, you’ll have some protection from the car’s engine. No matter what else, once the fight is on, do not stop. Just keep going and get out of the situation.

While on this subject, it’s a good idea to learn how to drive in reverse at relatively high speed. It will be easier if you forget your rear-view mirror on the windshield. Use your side mirrors and just turn the wheel the direction you want to go - keeping in mind that the direction you are going is "backward" from what you normally see out of your windshield. 

You can sort out the wreckage later as long as you’re alive and uninjured, so think of your car as an optional big battering ram and do some practice with maneuvering quickly. By the time your threat realizes what’s happened, your threat will no longer be threatening at all.