The Palm Pistol

Sorry, no pic, but it's the 21st centurys answer to the burning question:"What will a one-shot derringer look like in the Buck Rogers future?"

The answer is: Pretty silly. These guys are worried about accuracy:

Using the thumb instead of the index finger for firing, it significantly reduces muzzle drift, one of the principle causes of inaccurate targeting. Point and shoot couldn’t be easier.

At the distances one would expect to be using such a device, accuracy could probably be taken care of by making sure you press the thing firmly against your attacker before pushing the button. Anything beyond that would risk firing your one bullet and missing, which would likely enrage the attacker. Fortunately the barrel design looks smooth, so having it removed from your nether regions after a missed shot, should be easy and relatively painless.

For $300, you can get a real gun and some ammo to practice with, which will improve your chances of getting that much-desired hit on an assailant. Absent a hit, you can at least send him running in a hail of lead.