What is a class characteristic? Fired cartridge case example.
Class characteristics are the type of data that a firearm and toolmark examiner will use when they first start their comparison. These are generally pretty easy to see, and can be explained a viewed by a non-expert with little guidance. Sometime, as the example in the picture, it is as simple as a circle and square not being the same shape, or one being bigger than the other. Many of my elimination conclusions (a fired cartridge case was not fired by the submitted firearm) are from class eliminations.
Like the blog post about the bullet, sometimes there is not a firearm recovered. Much like the general rifling characteristics described in that article, the meaning of the class characteristics varies. A fired cartridge case (FCC) has many more toolmarks left on it from the firing process than the fired bullet. Generally with a fired bullet, an examiner is going to look at the land and groove impressions created from the barrel, which are frequently damaged from the bullet striking an object. FCCs are a bit different.
FCCs can come damaged, either being stepped on or ran over prior to being collected by CSI or responding officers. FCCs have so many different toolmarks to look at, this damage rarely makes the FCCs completely useless, and class characteristics are still observable. An FCC will have toolmarks created by (using a semi-automatic pistol as an example) the slide assembly, barrel, and ammunition magazine. Just from the slide assembly, you will have breech face marks, firing pin impression, ejector marks, extractor marks, and ejection port marks. You will frequently have, even though they are not always present, firing pin aperture impression with possible shear, firing pin drag, and slide scuff and drag. Depending on the make and model of pistol, you may have additional, even more discerning class characteristics.
Glock pistols and Smith and Wesson pistols always seem to have more valuable class characteristics, being able to narrow down the make and model of the pistol just by looking at the class characteristics and the caliber coming from the headstamp on the fired cartridge case (usually, otherwise you can measure the FCC). Earlier generations of Glock pistols had very rectangular firing pin apertures, which evolved into a more rounded rectangular aperture, and now they have a skull or avocado shaped firing pin aperture. The rectangular aperture is becoming more common however, as aftermarket Glock-compatible parts become very popular with the surge of Gen3-like 80% pistol kits. There are now many companies that are making Gen3 Glock-clones, which also share this rectangular aperture. Previously, for a short while, Smith and Wesson Sigma had copied this design before they were forced to stop.
Generally, a rectangular shaped firing pin from an aftermarket company does not have the same size and shape as a Glock pistol's when looked at closely, but the days of rectangular aperture = older generation Glock are gone. This is important information to know before investigative or court proceedings begin. How much weight should we be giving this evidence? A trained professional can help you figure that out.
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