Standard procedure for live guns on-set, nobody touches guns unless the armorer hands it to them. Also, when a live-gun is handled on-set, it is never pointed at anyone, it is aimed off-body.
In this case, a production assistant went and grabbed the gun for him to rehearse the scene with and the idiot actor aimed it at another person.
It should also be noted that, being single action, he actually needed to cock the hammer first before pulling the trigger. But it’s also possible that idiot actor was already holding the trigger down when he cocked it, thus why you never have your finger in the trigger until you are ready to destroy whatever/whoever the barrel is pointing at.
My theory is that he pulled the trigger and drew back the hammer with his thumb. His thumb then slipped off the hammer as he brought the weapon up and it fired.