Supposedly this was an unscheduled rehearsal, so the armorer was not aware or present to do her job.It's certainly possible that a sear could be worn enough to let go under very little trigger pressure (even just slightly back to the side). I would think it's the armorer's job to make sure this isn't the case.
People are saying that Baldwin is either lying and pulled the trigger, or that the gun fired itself. It's certainly possible that Baldwin has no memory of pulling the trigger: memory during stressful events isn't the most reliable...
...which is why I'm shocked there aren't multiple camera angles that show exactly what happened. On most film sets there's often multiple people running around recording everything that happens (good stuff for the DVD extras).
Several single safety constraints could have completely prevented this, like why was there live ammo on set?
It appears that multiple people dropped the ball and are collectively culpable.
It is hard to keep caring but the unfortunate filmographer is still dead.
JR