Rule #1 - gun beats no gun
If you left it at home, it is no use to you. Carry means you need to actually carry. A .50 hand cannon might be fun to shoot but it sucks as a carry gun.
Rule #2 - bullet that hits, beats bullet that misses
Only carry the gun you can shoot accurately. A .22 that hits beats a .44 magnum that misses.
Rule #3 - He who shoots first usually shoots last.
Carry only in an accessible manner. If you cant get it out and aim accurately and fire in less than 2 seconds you will likely loose in a gun battle.
Rule #4 - A stationary target is easy to hit. Learn to shoot and move, shoot and move
Rule #5 - The best guns and ammunition will eventually fail. Learn to clear stoppages rapidly.
Rule #6 - Even the best gun fights go bad for someone. Learn how to respond to gun shot wounds