From what I'm pulling out of my butt, I seem to recall that the FBI or whoever messed with the gun enough that they managed to get the hammer to drop on a live chamber without the trigger being pulled. I guess they got expert opinions that said the only way that can happen is if they really did screw it up in their investigations. Now, here's the thing with the modifications; there is an entire (small scale) industry devoted to slicking up cowboy style guns for both Hollywood and a sport called Cowboy Action Shooting in which a bunch of old parts dress up like they are in the old west and have shooting completions. They lighten the hammer springs up a bit, and polish all the surfaces where metal moves against metal. It makes a fast draw much faster for someone who practices. Hollywood sends their guns out to the same folks, because they also crave that historically inaccurate fast draw. His defense is likely to argue that the action job was done incorrectly and caused the hammer to drop without touching the trigger. Regardless, there is not a single source from any actual armorer from before or just after the incident that says what he did was standard protocol. I ha e found nothing but the opposite. And the law don't care if your SOP's say it's OK to point gun at somebody and pull the trigger.
All that said, he should probably get some sort of plea deal out of this. Punishing people for the sake of punishing them is dumb.