The rules are already about as dumbed down as they need to be, and your kid is probably smarter than you think. Most kids that age find it a lot easier memorizing stuff than grownups do.
Probably easier for him to memorize the rules verbatim, and for you to explain to him what they mean, than it would be for him to understand the very abstract concepts you're using to "simplify" with.
1) All guns are always loaded.
Five short words. The most abstract rule. Not too hard to explain though.
Explanation needed: no matter WHO tells him a gun is not loaded, he should always do only what he would do if the gun was loaded. (He'll probably get a kick out of being told not to trust an adult about this~! You can play with this point a little, and it's good if you do.) Make sure he knows that he doesn't get to say "but it's not loaded!" if he does something against the rules. The rules count ALL the time, not just some of the time.
2) Do not point the gun at anything you are not willing to destroy.
Thirteen words, none over two syllables. Again, all concrete nouns rather than abstract concepts.
Explanation: if you don't want to kill it or throw it away and never ever get it back, don't point the gun at it.
3) Keep your finger off the trigger until your sights are on target.
A dozen words. Again none over two syllables. Again all concrete nouns and no abstract concepts.
Explanation: "off the trigger" means to keep your finger all the way outside the trigger guard. You can put it straight alongside the frame.
4) Be sure of your target (and of what is behind it).
Five words, or eleven words. Hardest word is "sure" -- you can substitute "certain" or even "POSITIVE" with lots of emphasis.
Explanation: never, ever, ever shoot at a sound or a movement or at anything you cannot see. Don't shoot if you don't know where the bullet will stop after it goes through the target. Don't shoot if there could be a human being near where the bullet will land.
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Or were you talking about the Eddie Eagle rules?
If you see a gun:
1) STOP!
2) Don't Touch It.
3) Leave The Area.
4) Tell a Grownup.
Each of these rules is three words or less. Easy to memorize, easy to understand. Might have to explain who counts as a grownup (teenage babysitter??), but that's about it.
More about those rules here:
http://www.corneredcat.com/KidsAndGuns/FirstLesson.htm
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Or were you talking about rules for living? (APS material maybe ...)
pax