3.4 Trajectory Considerations for Sighting in a Gun The terms “sighting in,” “zeroing in,” or “zeroing” a gun all mean the same thing: adjusting the sights on a gun so that it shoots to point of aim at a selected range distance. Sighting in a gun always takes place on a target range, which may be an informal range in the countryside or an established range in some convenient location. The procedure for sighting in is familiar to almost all shooters. However, three questions arise frequently, and Sierra’s Infinity program can be used to answer them all. 3.4.1 Sighting in on a Short Target Range A shooter sometimes is faced with the following problem. He or she would like to sight in a gun for a zero range of, for example, 250 yards, but sighting in must be done on a shorter target range, say, 100 yards. The question is where should a group of shots be centered at 100 yards so that the gun will be zeroed in at 250 yards? The answer to this question is straightforward with Infinity. A trajectory is calculated using a zero range of 250 yards and using the altitude and atmospheric conditions at the target range. Then, the bullet path is read from the calculated trajectory for a range distance of 100 yards. This is the point where the group should be centered on a paper target located at 100 yards to assure that the gun is sighted in at 250 yards. Of course, any other pair of zero range and target range distances can be used. It is frequently necessary, for example, to use a target range distance of 25 yards to sight in a handgun at a zero range of 100 yards.