Greetings, first time posting to this forum. Regretfully this is a shameful situation that I hope all who read this can learn from. Earlier today I had a ND with my P22 in my home. It was pointed at an internal basement brick wall with no one but myself near and there were no injuries. I am hoping someone can provide some insight into what happened and also learn from my errors. Details: I removed my P22 from my range bag, ironically to mark the FIRE indicator red on the safety. Weirdly I have sometimes found the P22 safety to be counter-intuitive for me. For those that do not know, for a P22 when the S is showing it is SAFE, when the F is showing it is ready to FIRE. That seems obvious, but for whatever reason I would sometimes hesitate and have to consider if the correct positioning is move it to S to make it SAFE, and move it to F to make it ready to FIRE. Dumb, but something I was aware of and decided to mark the F with red paint to be sure. Anyway - I removed the pistol and dropped the mag. The mag was empty as expected (I just went to the range the week before). I checked the mag, and stated, "empty mag" per my normal routine. I pulled the slide back and visually checked the chamber. The chamber was empty. I stated, "Chamber clear." This is a habit I force myself to do...say it out loud. Lighting was adequate for visual inspection. I remember it cocked the hammer as it should so I must have went back far enough for that. I distinctly remember the chamber was visually empty. After that I went to find some red paint and could not find it anywhere. I never left the room. I went back to my gun and decided to put it away and do it later. As stated the hammer was back so I loaded the mag (need it inserted to decock), aimed towards an internal brick wall and pulled the trigger to decock it. BOOM. I was absolutely shocked. This is what I believe happened: When I pulled the slide back to check the chamber I did not pull it back far or with much force. It wasn't just a half inch or so - I did pull it back enough to set the hammer back - but I didnt pull it back with force. I am speculating but I think maybe the round was seated on the breech face, hidden from view at my angle, and I did not pull the slide back enough to properly engage the ejector? I also had the gun on it's side at an angle when I pulled back the slide. Honestly I do not understand the mechanics or how this happened. After - once I was done shaking and explaining to my family what the heck I just did - I tried repeating the failure to eject and I could not. I tried slow slide pulls and half slide pulls...no problems, either the round ejected or fell through the mag hole as it always does. Some failure points.... 1. I did not physically check with my finger there was no round in there somewhere. 2. I knowingly decocked the weapon on FIRE. This is just stupid and completely circumvented the last line of defense for something going wrong. 3. I did not rack the slide multiple times or with sufficient force before visually inspecting the chamber. 4. I did not look inside the chamber at all angles, a round was hanging up ready to reseat when I released the slide and I could not see it. I assume there will be those that believe I just did not see the round that was in the chamber. I understand how that is possible, but I personally do not believe this to be the case. I am intentionally mindful when I check...I literally think ok I am visually inspecting the chamber now, then state, "chamber clear", done. Any insight is appreciated. Be safe out there, thanks for reading.