Bullet-proof vests are expensive but fast food is dirt cheap. If one wanted to be able to protect themselves from stray bullets on the cheap just how fat would they need to get before they would not have to worry?
The gun has a 3m long barrel which you load a small ball bearing into. There is a valve between the chamber and the barrel which can be opened very quickly. The gas (at about 70 atmospheres) then expands extremely rapidly forcing the bullet out of the barrel at supersonic speeds.