Quote from Joe Doaks:
What I want to know is how he got that piece out and racked so fast. No small feat considering that it was concealed and he was sitting. I think I would still be gaping at the situation while he getting off the first two shots. Maybe because it was a sleazy internet gambling parlor he had already thought it out. He WAS against the back wall in sight of the door.
Like Phoenix alluded to earlier he is probably a vet. You can't really see him until he comes into the camera view, but the minute the guy with the pistol turned his back this guy got up and it looks like he tried to sneak as close as he could get.. nerves of steel.
IMO it was more of a reaction than anything else. Like you said he probably thought it through and was mentally prepared for that situation, and armed. Good for him.
