Why YSK: Even if you don’t own a gun, there is a chance you can encounter one at the home of a friend or family member. These are the four core rules of gun safety, but the same can be used with airsoft/paintball guns, nerf or even chemical spray bottles!
First, treat every firearm as if it were loaded. Even if it was clear the last time you saw it. If looking at a gun with someone else and they assure you it’s not loaded, you can respectfully ask them to clear it in front of you before you handle it yourself.
Never point a gun at anything you are not willing to destroy. Pretend there is a lazer beam coming from the barrel. Don’t let that “beam” point at anyone. That is known as “flagging”. In nearly all cases keeping the barrel pointing down between your feet is a good move.
Keep your finger off the trigger until you are ready to fire. This includes the whole trigger guard area. Most people keep their finger on the frame well above the trigger guard, pointing their finger in the same direction as the barrel. This is known as “trigger discipline”.
Finally be sure of your target and what’s behind it. Bullets can still travel a long way even after they pass through a target. If you are target shooting make sure there is a solid backstop. If in a real life situation you must positively ID the threat, no shooting at shadows or noises.
Maintenance should probably be in there too: A poorly maintained firearm is potentially dangerous to the user.
Doing proper maintenance is more applicable to someone who owns or is responsible for firearms. I think the rule for the casual finder would be adding something like “assume moving any part of the gun may cause something inside to explode”.
The rule for a casual finder would be to not touch it at all: If you find a random firearm, leave it the hell alone and report it to the authorities. It could be a weapon used in a crime and it’s probably not a great idea to get your fingerprints on it.