Zimmerman was taking a beating for 30-60 seconds or so. Did not go for his gun, yelled for help. While shifting his body so his head would be smashed into grass, instead of into the sidewalk, his shirt pulled up and his firearm became visible. At that point, says Zimmerman, Trayvon issued his death threat, "You're gonna die tonight." Trayvon was attempting to disarm Zimmerman so he could shoot Zimmerman. Probably both of them had hands on the pistol. If Trayvon pulls the gun toward himself, he very well could have "pulled the trigger" by his action. IOW, it may be that Zimmerman did not intend the firearm to discharge.
We don't know what he said under questioning, or why one of the investigators didn't believe the story. We don't have that affidavit to get a handle on the counter-argument.
or why one of the investigators didn't believe the story
Is there a reliable source that a particular investigator didn't believe Zimmerman's story, or is this based on the fact that the police report was merely titled "manslaughter"?
Some of our fellow Freepers that are better acquainted with the Kel-Tek have pointed out that exact scenario would explain the lack of a chambered round.