This list of rules is classic, and appears on numerous forums and websites. The actual author is unknown, and there are many variations.
Here is my favorite version:
1. Have a gun.
a. Preferably, have at least two guns.
b. Bring all of your friends who have guns.
2. Anything worth shooting is worth shooting twice. Ammo is cheap. Life is expensive.
a. Bring ammo.
i. The right ammo.
ii. Lots of it.
3. Only hits count. The only thing worse than a miss is a slow miss.
4. If your shooting stance is good, you’re probably not moving fast enough or using cover correctly.
5. Proximity negates skill. Distance is your friend. (Lateral and diagonal movement are preferred.)
6. If you can choose what to bring to a gunfight, bring a long gun… and a friend with a long gun.
7. In ten years nobody will remember the details of caliber, stance or tactics. They will only remember who lived.
8. If you are not shooting, you should be communicating, reloading and running.
9. Accuracy is relative: most combat shooting standards will be more dependent on “pucker factor” than the inherent accuracy of the gun. Use a gun that works every time. “All skill is in vain when an Angel pisses in the flintlock of your musket.”
10. Someday someone may kill you with your own gun, but they should have to beat you to death with it because it is empty.
11. Always cheat, always win. The only unfair fight is the one you lose.
a. “If you find yourself in a fair fight, you didn’t plan your mission properly.”
12. Have a plan.
a. Have a back-up plan, because the first one won’t work.
13. Use cover or concealment as much as possible.
14. Flank your adversary when possible. Protect your own flank.
15. Don’t drop your guard.
16. Always perform a tactical reload and then threat scan 360 degrees.
17. Watch their hands. Hands kill. (In God we trust. Everyone else, keep your hands where I can see them.)
18. Decide to be aggressive enough, quickly enough.
19. The faster you finish the fight, the less shot you will get.
20. Be polite. Be professional. But… have a plan to kill everyone you meet.
21. Be courteous to everyone. Friendly to no one.
22. Your number one option for Personal Security is a lifelong commitment to avoidance, deterrence, and de-escalation.
23. Do not attend a gun fight with a handgun whose caliber does not start with a “4.”
a. Nothing handheld is a reliable stopper.
24. Carry the same gun in the same place all the time.