A couple of days ago I watched a fellow sight in his rifle for a competition shoot. He laid the gun on a nice, hard sandbag, took the caps off the scope turrets, and fired a shot. Then he cranked the dials. He did this ten times in a row: blam, crank, ka-pow, twirl, bang, twist. Eventually, the barrel was hot enough to fry eggs on, and the rifle was shooting where he thought it should.
Later, by sheer chance, I got to score his target, and he was all over the place. He had one good score which was pure luck, followed by a bad score, which was inevitable because he broke the rules of sighting in, which are:
One shot tells you nothing. You need a three-shot group, and you proceed from where the center of that group is.
Don’t let the barrel get any hotter than it’s going to get in actual use. If you’re hunting, that means you sight in with a cold barrel. If you’re going to be shooting a five-shot string, that’s as many shots as you can take before you let the gun cool off.
Your rest should be what you’re actually going to use. Shooting over a nice hard sandbag may not give you the same zero that you’ll get get by shooting over your hand, or with a sling, or with a bipod.
And of course you never, ever, sight in with one brand of ammo and bullet weight, and type of bullet and then switch to another. But I don’t have to tell you that, do I?