Yes, Garrett should not have overlooked that likely scenario.
What concerns me is that I have yet to see it proved that ejector pin marks are unique. If they are, then the prosecution will have to prove that to the jury. If they are not distinct enough to say with 100% certainty that the bullet was cycled through RA's gun, then the prosecution's case becomes quite weak indeed—unless, of course, there's compelling evidence against RA that was left out of the PCA.
The whole thing with cycling scratches acting as gun fingerprints was news to me until the PCA came out, so I don't know how that's going to play.
I tell you though, if I'm on that jury, the defense has a wall to climb before they can claim reasonable doubt. RA was there on the bridge then, by his own admission, and no witness puts more than one male there then.
All the witnesses describe someone who looks like RA. Someone like him walks in determinedly with his face obscured. Libby makes a video of someone like him, the only male known to be there, walking up to her and Abby.
One of the girls says "gun," so we can figure a gun was used to move them down the hill.
An hour and 40 minutes later, someone like RA walks muddy and bloody away from the scene and toward RA's car.
Somewhere in here, Occam's Razor is screaming at me, and we haven't even got to the bullet yet. The pieces are all fitting and it's getting unreasonable for it not to be just what it looks like.
Then we have a .40 round at the scene. I don't think it was left as a sick-o calling card. In that case he would have left it sitting directly ON one of the victims. It was unintended. But we DO know a gun was used, and here's this thing between the victims.
So then the prosecution says it can only have come from RA's gun out of all the .40 handguns on Earth. Just the icing on the cake for Me, The Jury.
Then the defense furiously spends three hours attacking that icing on the cake. It's not science, they say.
Maybe they do a half-decent job creating reasonable doubt about the extractor / ejector scratches being a unique signature. I'm probably unmoved. He was there. He was the only male there. He was all of those men, including Mr. "Guys... Down the hill" in the video.
Anyway, what are the odds somebody other than the killer lost a .40 bullet in just the location where later on some guy who used a gun to kidnap two girls leaves them dead? Oh! The cartridge is a semi-unusual caliber. And the guy who admits being there but didn't do it owns a handgun, which he never lends out, of that caliber? Pull the other leg!