My thoughts exactly. When guns come off the assembly line they are pretty darn similar. Over time the extractor and other features of the chamber progressively become more individual depending on how many times it's fired, what kind of ammunition is fired through it, how often the gun is cleaned, etc.
So how many times has RA's .40 Sig been fired in the last 5 1/2 years? 10? 100? 1,000? How similar is the chamber, the ejector, the extractor today compared to 5 1/2 years ago? An expert is going to have to admit these features change with use progressively over time. That's how guns become "individual" in the first place. They grow into themselves.
I go to the range about once a week on average and fire about 100 rounds. I myself am now wondering how similar my firearm is today after firing thousands of rounds through it. I've never really thought about it, but I sincerely doubt it is *exactly* the same as it was 5 years ago. The barrel itself, maybe, but the components of the chamber? As a juror I would be more comfortable if that weapon and casing were matched nearer the time of the crimes rather than 5 years later.
But who knows? Maybe the prosecutor will get lucky and RA hasn't fired that gun at all in the intervening years. Some people don't. They buy a firearm for personal protection but rarely, if ever, fire it.
Another thing I would want to know as a juror is how many other casings, spent or unspent, were found on RL's property. RL had a number of firearms too and it's likely he and other people over the years carried out target practice on that property. It's kind of what you do in rural America. Were there other .40 casings found there? How many? Were any others "similar" to RA's .40? Were there 10 other .40 casings that were similar but you're not telling me about the other 9?
As a juror, I have a lot of questions about this piece of evidence and I hope the prosecutor is prepared for them. My opinions only.