If there was an empty chamber, an ejected shell, and a non-empty magazine, that indicates that either Zimmerman took out the chambered round before handing over the pistol (good firearms practice), or that the gun’s slide went back far enough to extract and eject the spent shell, but not far enough to catch and chamber the next round from the magazine.
Then Z would have handed his gun over to the police officer with the slide jammed open, which surely would have been noted. Either way, there’s no big mystery, or much that would affect our picture of what happened here that I can think of right off. Maybe a Lawyer could!
The expended brass was still in the chamber. Gun never cycled, indicating Trayvon had a strong grip on it, preventing the slide from cycling.
Someone pointed out that seizing a gun like that is a grappling skill taught in prison, among other places .... it will absolutely prevent a revolver from firing.