Accuracy is a must for the hunter. No one wants to wound prey, or leave it to a slow, painful death. Hunters want as clean a harvest as possible. But a hunter who is the best shot in the world can't be accurate if his rifle isn't freshly sighted-in. A properly sighted-in rifle ensures enough accuracy with the bullet's trajectory that a hunter can centre his cross hairs on a target and not fear a miss, or, Heaven forbid, a crippling shot.
The general rule of thumb for sighting in is to set up your rifle and scope to shoot two inches high at 100 yards. These means a 100-yard shot aimed accurately at the centre of a deer's vitals will strike two inches high at the peak of its trajectory — still well within the vital area. The shooter then determines the range at which the shot strikes three inches low. That is the maximum range at which the hunter can fire without holding over his target.
It's important to simulate as many of your hunting trip's variables as possible to sight-in accurately. Use the same round you will be using on the hunt, and, if possible, the same manufacturing lot. Sight-in your rifle near the time of year you plan to hunt. The temperature of your gun's barrel affects its accuracy. Try to sight-in on a windless day.

Begin the process by bore-sighting your rifle. Make sure the bore and the cross hairs of your scope are aligned on the same point at 100 yards. This is not difficult with a bolt-action rifle. Simply remove the bolt so you can peer down the bore. With other actions, a mirror arrangement may be necessary and these can be bought inexpensively from a sporting goods store. Clamp the rifle in place, or position it solidly, so that the bull's eye of the target at 100 yards is centred in the bore. Then adjust the scope to place the cross hairs in the same position. Check the centering of the bore again. It may require four or five adjustments to finish the job. Position your self carefully when you begin to shoot. Make sure nothing but you is touching the rifle. Find a comfortable firing position, and support your forearm and elbows firmly.
Centre your cross hairs on the bull's eye at 100 yards. Fire at least three and no more than five shots, about a minute apart (you don't want the barrel to overheat and throw off your shot). Correct your sights and fire another group after your gun barrel has cooled. Repeat the process until the average point of impact in the group is over the bullseye and two inches high.

Let your barrel cool completely, then repeat the group of shots at least once to confirm you are hitting two inches high and directly over the target centre. Shoot groups at intervals longer than 100 yards until you determine the ranges at which you are striking three inches below the bullseye. That would be your maximum range for firing at a target without holding over. Depending on the cartridge and firearm, this will likely be between 200 and 250 yards. Shoot groups at longer intervals to determine where the bullet strikes six inches below the bullseye. This is a reference for shooters who are experienced not only with shooting, but with estimating range. This is the maximum range at which an expert marksman should try a shot, holding over the target by six inches.