Measure wound cavity volume

If the bullet exited the Test Tube you will need to take your thumb and push some of the expansion material into the exit hole so that it is completely closed.  Then, pour water from a measured container into the bullet entrance hole until the Test Tube is full of water.  Make a note of how much water was required to fill the Test Tube.  This is the volume of the wound cavity.  You can then pour the water from the Test Tube.  (You will need between 300 and 500 ml of water to make most measurements.)