The problem with shooting a gun at someone is that you just can't control how fast the bullet is going. More than likely, it's going very fast and will seriously hurt or kill them. But geez, you didn't want that! You just wanted to kind of hurt them a little to teach them a lesson. Don't worry, my violent friend, science is working on a solution for you.
The solution will be a gun that controls how fast it fires bullets. Want to blast through a wall? OK, crank it up to full power. Want to just give someone a really bad bruise? OK, turn it down a bunch. Want to scare the crap out of someone by aiming a gun at them and firing a bullet that falls to the ground at their feet? Kind of a jerk move, but that's possible too.
It's all based on the technology behind toy rockets, of all things, and it works "by mixing a liquid or gaseous fuel with air in a combustion chamber behind the bullet. This determines the explosive capability of the propellant and consequently the velocity of the bullet as it leaves the gun." Cool.
Via New Scientist