Zero (Sighting In)
Also known as: sight-in
The distance at which a gun's sights are adjusted so the point of aim matches the point of impact.
To 'zero' a firearm is to adjust the sights or optic so that the bullet strikes exactly where you're aiming at a chosen distance. Because bullets travel in an arc, a gun is only dead-on at the zero distance (and often a second, farther crossing point); beyond that the bullet drops, and very close in it may hit slightly low. Common rifle zeros include 50, 100, and 200 yards depending on the cartridge and use.
Choosing a zero is about matching the trajectory to your typical shooting distances. A 50/200-yard zero, for instance, keeps an AR-15's point of impact within a few inches of the line of sight from muzzle to 200+ yards. After zeroing, a ballistic chart or DOPE card tells you how much to hold or dial at longer ranges.