Boresighting

aka "Saving a lot of ammo with a new sight".

Imagine if the scope/red dot/iron sights on a rifle get damaged and have to be replaced. If you're in a nasty situation (like an emergency), you don't want to spend any more ammo than necessary to get that new sight zeroed in. Boresighting is a way to do that.

Note: this only works on a firearm where you can remove the bolt (bolt-action rifles and some semi-auto) or the bolt group (AR-style rifles) to see through the barrel.

1. Have a spot selected as your aiming point, somewhere from 25 to 50 yards away. I've got a round piece of wood painted bright yellow that I can stick on a fence.
2. You'll need a solid, stable place and way to set the firearm up. A table or the hood of a car will do, with a couple of sandbags to hold the gun steady.
3. Clear the weapon of any ammunition, then get it ready. In this case let's say you've got an AR that needs a new scope mounted, so you will need to take the upper receiver group off the lower, remove the bolt carrier group, and then set the upper onto your stable rest.
4. Look through the bore and line it up with your aiming point in the center. Take your time. Then look through the sight, and adjust it so that it's on the aiming point. Look back through the bore in case anything shifted while you were fiddling with the sight and adjust if necessary. Repeat as necessary.
5. When you've got them both dead-on, walk away for a couple of minutes to rest your eyes, then check both again. When you're sure both are on, you're ready to put the rifle back together, go to a suitable place and shoot to do final adjustments.

Do it well, and at the least you'll be on the paper and pretty close to zeroed, which means less shooting time and less ammo used in getting the final adjustments made.