Guns guns everywhere, and not a drop to drink.

Using a firearm is repetitively simple. Like using any weapon or tool. Assuming the gun is primed and ready, you just pull the trigger. The hammer hits the primer causing a violent chemical reaction that leads to a controlled explosion. An instant later a projectile (or many in the case of buckshot and the like) emerges from the barrel traveling hundreds of feet per second. The projectile(s) travel until their energy is spent.

Depending on the charge and shape/mass of the projectile and the amount of powder involved, they can even penetrate substantial quantities of matter before coming to a rest.

Fun stuff huh?

I grew up in Alaska. You would think that I would have learned all about firearms(guns) with my mother's milk. And after a fashion you'd be correct. Given time, I think I might be able to figure out how to figure out most weapons, but I still don't know much in the realm of cleaning and maintenance. These are important factors. If you don't take care of your tools properly they tend to fail. And as a complex machine, the parts in most firearms tend to wear with time. A broken gun is only really useful as a club.

On the whole shooting is easy. A. Point weapon. B. Squeeze trigger. Repeat as necessary until you either run out of ammunition,or targets. Or are yourself killed/neutralized. Modern industry has made it so. Or rather, shooting is easy to learn and more challenging to master. Assuming that you actually want to hit what you're aiming at. That takes lots of practice. And a decent understanding of physics.

The first concern is a common one. Gravity. Eventually the speed of the projectile is going to be overcome by wind resistance and the pull of gravity and the bullet will be fall back to earth. How fast this happens, depends on the weapon. The M-16 tends to have better range and accuracy than the rival AK-47. I don't recall if this holds for the updated AK-74 or not.

Then there's the wind, it has a tendency to alter the path of the projectile.

All you really need to do is line up the sights, and adjust for wind and gravity.

This of course is all fine and good, when you're shooting at a paper target. Not so much so when the target is shooting back. So, get some practice.

Really, why bother talk about this one? Guns will probably come in handy no matter how civilization falls. More so with some than others. At least in the short term before the stockpile of ammunition is blown through. If you learn to shoot straight, you'll use less ammo, and your stash will last that much longer. Then you'll need to fall back on clubs and pointed sticks and what-not. Assuming you survive that long.

One shot, one kill.