Why couldn't it be the source?
(unspent round ejected at the "GDTH" point couldn't be the source of the round found with the bodies unless BG did it deliberately, which I implied in my message but didn't explain; your msg respectfully snipped for focus.)
Sorry, this has probably been answered better by others; I'm unable to keep up with the number of messages lately. But:
Just personally, it
seems likely to me that BG may have racked the slide to intimidate the girls. It's certainly a frightening sound if the gun is pointed at you! And if he did do that, it was
at the end of the bridge when he said "GDTH," and it MIGHT (IMO) be the "gun sound" rumored to be on the BG audio. (And it might well NOT be the 'gun sound,' LE may know but we don't.) We know "he's got a gun" is on the audio, so he WAS using the gun to scare the girls. Racking the slide would emphasize the gun's menace in the girls' minds, I'm sure.
Brief digression: everybody I know who carries a pistol carries with a live round in the chamber and the safety on. That's just how you do it if you might actually NEED to use the pistol. He couldn't shoot it at the trails without possibly alerting people nearby--that .40 caliber pistol would be loud. But also IMO he certainly wasn't on the trails with an unloaded pistol; that would increase his own danger. As you said yourself, if you might need a pistol, you carry it loaded, QED.
SO: If there was a clip in BG's pistol at the end of the bridge, when he said DTH, and a round in the chamber, racking the slide (which certainly would intimidate the girls) would eject the unshot bullet and chamber the next round in the clip (magazine.) If he DID do that to scare the girls, it would be a really good idea to pick up that round, though who knows what's going on with a murderer abducting innocent kids? Maybe he wanted to but was busy, maybe he was too full of excitement to think of it ...
Here at last is my own answer to your question: This intimidation at the initial contact (a scare to accompany "G,DTH") means if a bullet was ejected,
it is (again IMO) not the bullet that was found between the bodies, which was far away from the end of the bridge and long after the girls had been intimidated into compliance, right? Unless it was deliberately picked up and brought to the murder scene. Since the audio ends after "GDTH," a 'gun sound' at the murder scene, after crossing the creek, would not be on the audio. So the round at the bodies would not have been from racking the slide at the end of the bridge, 200 yards (or whatever) away--
unless BG deliberately picked that round up and later placed it with the bodies as a bizarre murderer's signature, a way of memorializing his loathsome kill.
He might have racked the slide at both places (bridge and murder location) for intimidation, and he picked up the unspent round at the end of the bridge, but not at the murder scene, for whatever reason. Possible IMO but less likely.
Which suggests to my own underpowered brain that the cartridge found with the bodies is more likely to have been a "calling card," a signature. MHO, of course.
I'm so glad there's finally an arrest in this case! Now that I know the wheels of justice are in motion at last, I feel like I can relax a bit, if you know what I mean. The girls deserve a proper and just conclusion to the case, IMO.