Just sharing Jason Goodman's interview with Redacted.
He posits that Charlie was shot from behind with an air pistol, with the projectile traveling at 500 feet per second. He highlights 3 dots on a video that suggest the speed and angle etc and that it was filmed in high definition at 240fps.
Now, I was going to just share the entire thing - but I'll share it at the relevant timestamp.
Apparently, on viewing the footage, the projectile doesn't travel towards Charlie, but away from him.

He goes on to say it's an artifact of compression, blah blah....
Hilarious.
So, we have the theory (explosive mic), the C's confirmation, and now high res/frame rate footage of the projectile leaving his neck and traveling away from him.
Beyond some of the technicalities (such as how exactly the device didn't move backwards when the projectile was fired? it's not impossible to engineer though), that all looks pretty solid to me, as to how he was killed.