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I took my pistol to the range one day and discovered that it is necessary to remove the o-ring if a person expects the gun to fire!
Tell me about it! I had it happen to me (or did I do it to myself?) on my second trip to the range with my brand new P-01. I started freaking out thinking that I'd inadvertently messed up my brand new pistol by dry firing it too much. With the dim lights in my lane it took me a good few seconds to notice this small black O-ring sitting in the black recess of the black slide. Good it was happening at the range and not while my home was being invaded by zombies.

BTW, somewhere on the net, I've read a good advice about dry-fire drills. I do not remember where, so I cannot acknowledge the author, sorry. You should start your dry fire drills by saying out loud “I’m going to start my dry-fire drill" and end your practice with, again, saying out loud “I’m finished/done with my dry fire practice". At first I thought it was silly to talk to yourself, like Harry Potter saying some magic words. Like this was going to make any difference? But the more I think about it, the more I see the wisdom of it. By saying these words you are forced to focus on your actions, kind of put a boundary around your practice, so 15 minutes later you do not pick up the gun (which, BTW, you've already loaded with live ammo) to do just one more dry-fire practice at your TV.
This spoken statement about finishing the practice would be a good time to make sure that your gun has snap caps, O-rings, foam ear plugs, etc, removed! It is for me now!