"Those who will have an ND in the future, and those who will be vigilant enough not to."
The word "vigilant" is a nice choice here. Every time you pull the trigger, you are testing your knowledge of whether or not there is a round ready to be fired from the gun. "I'm pretty sure" is not the standard. The vigilant owner will
know for certain.
Here's an example:
Yesterday I removed the magazine disconnect safety from my LC9, and wanted to show the result to my neighbor, who works on guns as a hobby. He was curious. So I did this:
1. Took the magazine out of the gun
2. Worked the slide, ensured the chamber was empty
3. Inspected the box where I'd stored the rounds - counted 8
Then I took it over to show him. When I handed him the gun, it was pointed in a safe direction. I told him it was unloaded. As usual, he:
1. Removed the magazine
2. Worked the slide and ensured the chamber was empty
Then he kept the muzzle pointed in a safe direction and tried a dry fire to see how the mag disconnect procedure had been accomplished.
In other words, it isn't "just checking." You layer safety in successive procedures and habits, doing them religiously, regardless of "common sense." Even when you know for sure the gun is empty, you still check the chamber one more time before pulling the trigger, because it's an automatic habit.
The primary way that NDs happen is through carelessness. Getting sloppy. Being complacent. Yet, familiarity does not have to breed contempt. Every time you handle a firearm, you have a Golden Opportunity to yet again ingrain good habits and institutionalize in your memory the rules of safety.
It is wrong and insulting to say that people who spend a lot of time with guns are more likely to have an ND, because they are firing 100,000 rounds, or a million, or whatever. If anything, the more you shoot, the better your procedures should be.
We don't expect the airline pilot to occasionally crash the plane.