The gas chamber which is filled by high pressure helium

The bullet  - a small steel ball bearing.

The perspex tube full of gelatin to act as a target

This was mounted in front of a backstop full of rags.

First the gas in front of the bullet hits the gelatin

The bullet hits the gelatin at about 500m/s, that is over 1100mph - nearly twice the speed of sound!

While the bullet is passing through the jelly it creates a conical shaped hole behind the bullet.  This is because to get out of the way of the bullet the jelly has to move very fast, and once the bullet has passed it will keep moving in the same direction expanding into a cone.

Bubbles form in front of the bullet, this would not happen in reality, it is because the perspex tube is stretched where the bullet hits it, and this stretch travels down the perspex as a wave faster than the bullet is moving, so the gelatin is stretched forming cavities.

It all gets too much for the perspex which shatters. The bullet leaves the Gelatin at about 180m/s. This means that by a very rough approximation you would have to have twice this much fat to have stopped a supersonic bullet. That is 72cm of fat and is somewhat unfeasible, We think that bullet proof vests are probably more practical and probably cheaper!