Remember, here's the situation:
A bullet is removed from the body of a victim. It's placed into evidence and locked away. A gun is found on a suspect, who's now a hundred miles from the crime scene. The gun is also placed into evidence and locked away.
The essential question this time is: was the bullet from the victim fired from this gun? If so, that's some good evidence. If not, we need to know that too, so we can find the right murder weapon.
So, APD firearms guru Greg Karin retrieves the bullet and the gun from the evidence locker and retreats to his lab.As before , he fires the weapon into a barrel of water. He'll do a number of test fires, fifteen for example. Then he will put the bullet from the victim under a microscope, examine it, and take close-up photos. He will do the same with the test-fired bullets.
This is what he'll see:
Now, you'll see reference on these photos to two types of characteristics: class and individual. The examiner is looking to make matches in both classes, and here's their basic definitions:
Class characteristics can be defined as:
Intentional or design characteristics that would be common to a particular group or family of items.
The class characteristics of firearms that relate to the bullets fired from them includes the caliber of the firearm and the rifling pattern contained in the barrel of the firearm.
Where individual characteristics are:
marks produced by the random imperfections or irregularities of tool surfaces. These random imperfections or irregularities are produced incidental to manufacture and/or caused by use, corrosion, or damage. They are unique to that tool and distinguish it from all other tools.