I never understood why the gun went off when Peter had supposedly tested them all. Do we assume he fired all the guns once but there was a bullet in the next chamber?

Or not? Because Ginty then tells Nicola that "nothing would have happened" if Nicola hadn't bashed Patrick's wrist (and Nicola herself seems to think that Lawrie is the one that would have been shot, she doesn't mention Patrick) but surely if Nicola hadn't done anything then, as legionseagle says Patrick would have killed himself?

Please explain, anyone who understands guns.

Or is there even meant to be something a bit supernatural about it all - harking back to thate earlier Devil on the Rooftop stuff?