Thanks - Not one I am familiar with (only handgun I have ever had was an issue Browning 9 mm), but yeah, sounds like a combination of residue blowback from a target closer than 3 feet away and the partially open slide.
The L96 or AS50 are more my tools of choice, though they are significantly more difficult to carry in your waistband!
Re: the partially open slide ...
There were gunshot residue patterns and cuts on Trayvon’s hands, which seems to indicate a struggle for the gun. If Trayvon’s hand interfered with the slide mechanism, that could explain why another round was not chambered.