how about rule number five is "Always check for obstructions in the bore before shooting."
IMHO - This an extremely confusing addition to the "4 Rules".
-Do you check for obstructions between each shot? What if you are shooting an autoloader or a "machine gun?".
-Do you suspend disbelief that you should "treat every gun as if it were loaded"? We, of course need to do this for cleaning, but it is not a requirement to suspend this disbelief when shooting.
-The method to "check for obstructions" is going to be different for many kinds of guns.
-If this was a "rule", published on the Internet and printed in gun manuals, it would probably cause a LOT of first-time shooters to eyeball the muzzle of their gun - and probably cause a death or two.
The Four Rules:
Rule # 1 - Treat every gun as if it were loaded.
Rule # 2 - Never let the muzzle of a gun point at anything you do not want to destroy or kill.
Rule # 3 - Keep your finger off the trigger (pointed straight forward) until ready to fire.
Rule # 4 - Be absolutely sure of your target, and what is behind it.
These rules don't account for an obstruction in the barrel, but they also do not account for the wrong sized round in the chamber, wildly inaccurate sights, dropping guns, handing guns to known idiots, etc.
To be honest, I have rarely checked for an obstruction in a gun. I do check with a new gun and, of course when cleaning and if I ever have a strange sounding or feeling discharge.
It seems to me that the four rules are just fine. Any addition would include bigger fish to fry...like a rule that you open the chamber when handing a gun to someone, or you empty your chamber when coming upon someone in the woods.
The thing is...MY new rules (and YOUR new rule) apply only in a peaceful situation.
Jeff Cooper's Four Rules apply even when someone is shooting at you and you need to fire back! I am guessing he thought about this long and hard while writing them. Hence the belief in the wisdom of Col. Jeff Cooper.