...that's how I was taught many years ago.
Where did you learn this procedure?
I was curious to see if maybe I was off base, so I just finished going through 20 some-odd owners manuals for firearms with detachable magazines.
Every single one of them recommended removing the magazine first and only checking the chamber after the magazine had been removed.
I rather rack the slide back to see if there is a round in the chamber first and then remove the mag.
The issue is that with the magazine in the gun, one can check the chamber and then drop the slide and reload the chamber that was just verified to be empty. Checking the chamber while there's still a loaded mag in the gun creates a situation where the chamber check can be done properly and an empty chamber verified but then the chamber can be loaded inadvertently from the loaded magazine that's still in the gun.
If the mag is out, then no matter how the procedure is scewed up from there on out, at least the chamber won't actually be reloaded. One might not properly check the chamber (which would be a fail regardless of what order the process is performed) but at least once a proper chamber check is performed, there's no way to chamber another round once the mag is out of the gun.
...If they had locked back the slide first and the round came flying out, they would then remember to drop the mag.
And if the chamber was empty and nothing came out then by that logic they might not remember to drop the loaded mag and when they dropped the slide they would chamber a round.
People who rack the slide and see an empty mag might unwittinly assume that the gun is empty (unchambered). An empty mag might lead to this assumption and they would more likely to forget step 2 (rack the slide to check).
I don't understand how people who rack the slide are more likely to forget to rack the slide.
...so if you forget step two in your senario, then you could just as easily have a ND because step two would be to eject the round. This being the most common occurances of ND's.
I'd be interested to see the data on that--what study are you quoting?
Anyway, the bottom line is this: In every applicable owner's manual I could find, the proper procedure is as follows:
1. If there is a safety and/or decocker control then put the gun on safe and/or decock it using the control.
2. Remove the magazine.
3. Operate the action and verify that the chamber is empty.
If you have (or know of) a manual that specifies a different procedure, I'd be interested to see it.