"Bullet That Killed Charlie Kirk Doesn't Match Tyler Robinson's Rifle..." But what does this actually mean?

You look at your phone notifications and see this headline from People Magazine authored by Liam Quinn. The subheading reads, "Robinson's lawyers claim an ATF analysis failed to link the bullet to the rifle..."

Reading this with no guidance, this seems like a case-closing piece of information. How can you continue to trial when the lethal bullet does not link back to the firearm of the suspect? Continuing to read, you come across the "need to know" section. "...lawyers are arguing in a new court filing that the ATF has been unable to conclusively link the bullet used to kill Charlie Kirk..." It's not looking good for the prosecution, at least with no one able to interpret what this forensic language actually means.

At the surface, it does not look good for the prosecution. I agree that not having a conclusive link between the lethal bullet and the suspect's firearm introduces doubt. But how much doubt?

In the world of forensics, a conclusive link would be coming to the conclusion, based on the data in your analysis, that one item is linked to another. A same source or identification conclusion in the world of firearms and toolmarks. The definitive conclusion in the opposite direction is a different source or elimination conclusion. In this article, they have an "unable to conclusively link" conclusion. This leaves two possibilities. One, the bullet is so damaged from the high velocity impact that no useful information can be obtained. In this case, the bullet is so damaged, there are no useful toolmarks left by the rifle's barrel, likely to the point that not even a nominal (general category) of bullet caliber (size) can be determined.

Reading between the lines, my educated guess would be an inconclusive conclusion. These conclusions occur when some information can be obtained, such as a nominal caliber range for the bullet, and that information that can be obtained does not rule out the suspect firearm (elimination conclusion). However, and importantly, it also means there is not enough information present to make any definitive conclusion. This leaves inconclusive conclusions in a strange place, where some doubt is introduced, but it also does not completely eliminate the suspect firearm. In my opinion, this is why no one in the legal world likes or uses inconclusive conclusions in court. It gives no firm answer either direction. I would argue it is more favorable to the defense though, and I am surprised I do not testify to more inconclusive conclusions. An inconclusive does introduce more doubt, especially if the inconclusive is tied to a very common caliber, like 9mm Luger caliber. Depending on the population in your region, there are likely hundreds or thousands of 9mm Luger caliber firearms, for example.

For the prosecution, the argument is how common it is for a projectile (bullet) from a rifle that hits a substrate to retain enough information to make a source conclusion (identification or elimination). In my experience, especially with some designs of projectiles, high-velocity projectiles frequently break apart or become so deformed as to not be useful. This does not mean you cannot get any information, but high-velocity projectiles tend to be a difficult comparison. I would not be surprised at all to find a projectile from a rifle in an unusable state.

So, while the forensic content is not actually as scandalous as the headline suggests, it does indicate that the information from the lethal bullet ranges in usefulness from limited to not useful at all in providing certainty beyond a reasonable doubt.