U.S. News

conspiracy-brained people are all seizing on this as evidence that supports whatever conspiracy theory they already believed about Kirk's killing.

but really, this is not surprising at all. from 2022: The Field of Firearms Forensics Is Flawed: The matching of bullets to guns is subjective, and courts are starting to question it because of testimony from scientific experts

Skepticism of firearms identification is not new. A 2009 National Research Council (NRC) report criticized the firearms identification field as lacking “a precisely defined process.” Guidelines from the Association of Firearm and Tool Mark Examiners (AFTE) allow examiners to declare a match between a bullet or cartridge case and a particular firearm “when the unique surface contours of two toolmarks are in ‘sufficient agreement.’” According to the guidelines, sufficient agreement is the condition in which the comparison “exceeds the best agreement demonstrated between tool marks known to have been produced by different tools and is consistent with the agreement demonstrated by tool marks known to have been produced by the same tool.” In other words, the criterion for a life-shaping decision is based not on quantitative standards but on the examiner’s subjective experience.

also, pay attention to the wording:

could not conclusively connect a bullet fragment recovered during Charlie Kirk’s autopsy to the rifle

they're not saying they concluded there's no link between the bullet and the rifle. they're saying they can't conclude there is a link. absence of evidence is not evidence of absence.

the actual news here is simply that his defense attorneys asked for a delay to the trial, so that they have time to prepare to rebut all the possible evidence the prosecution might present.