In order to understand the science of properly sighting in a rifle, we must first understand a thing or two about the effect of gravity on a bullet fired from it.
First, visualize a rifle with its barrel held parallel with the ground. Now, picture a bullet held in the hand, adjacent to the rifle barrel. If a bullet held next to the barrel is dropped at precisely the moment the other bullet exits the muzzle of the barrel during firing, both bullets will strike the ground at the same time. This is due to the fact that the gravitational pull on the two bullets is the same.
Understanding Line Of Sight
If, while aiming a rifle at a target, someone stretches a tight string from the center of your eye to the point on the target at which you are aiming, the string would represent your line of sight. Regardless of how far away the target is, your line of sight will be as straight as a new banjo string. If the rifle you are shooting is sighted in to fire its bullet parallel to your line of sight, gravity will begin to pull the bullet downward and away from your line of sight immediately after it emerges from the barrel. To optimize the effective range of a rifle its sights must be adjusted so its muzzle is angled slightly upward. This will shift much of the bullet’s travel above the line of sight.
The bullet will cross the line of sight twice: First, at close range (usually 25 yards or less) as it travels upward, and then farther downrange as it curves back to earth. The more the muzzle of the rifle is elevated in relation to the line of sight the farther downrange the bullet will travel before it crosses your line of sight the second time. Carefully adjusting the relationship between the sights of a rifle and the angle at which its muzzle is elevated is called sighting in or “zeroing” a rifle.
Rifles with open sights are usually sighted in by adjusting the rear sight. The rear sight is always moved in the direction you want the bullet’s point of impact to move in relation to the line of sight. If a group of bullets lands to the left and below your line of sight, move the rear sight up and to the right.
Moving the front sight causes the opposite to happen. In the case of a rifle shooting too far to the left, the front sight would need to be shifted to the left. If the rifle is shooting too low, the height of the front sight needs to be decreased, assuming that rear sight height remains the same.
A rifle with a peep sight mounted on its tang or receiver is easier to sight in because its adjustments are more precise. Most also have directional arrows on their windage and elevation screws to help simplify adjustments.
The simplest of all rifle sights is a telescopic sight. The knob at the top of a scope is used to adjust elevation (or to cause the line of sight to move up or down in relation to the bullet’s point of impact).
The windage adjustment knob causes the line of sight to move left and right. If the group you shot landed too far to the right, you would twist that knob in its “Left” direction. How far the line of sight will be moved when one of the knobs is turned one click or graduation depends on the scope. There are exceptions, but most scopes designed for big-game hunting have 1/4-inch adjustments. This means that when either knob is turned one click (or one graduation), the line of sight will shift 1/4-inch at 100 yards. That same amount of adjustment will cause the point of aim to shift 1/16-inch at 25 yards, 1/8-inch at 50 yards, 1/2-inch at 200 yards, 3/4-inch at 300 yards and so on. If you want to cause your rifle to shoot two inches higher at 100 yards, turn the elevation knob in its “Up” direction eight clicks.
The click or graduation value of many scopes can be off a bit, so always fire one last group for verification after the final windage and elevation adjustments have been made.
Some hunters zero their rifle to be dead-on point of aim at 50 or 100 yards. This is fine so long as shots are not attempted at greater distances. Some calibers are best zeroed dead on at 100 yards simply because that distance represents the maximum practical effective range of the cartridge.
I prefer to zero most rifles to handle the longest shots practical for the cartridges they are chambered for. More than four decades of big-game hunting on five continents have convinced me that three inches high at 100 yards is the best way to zero most modern centerfire rifles.
When a typical .30/30 rifle is zeroed dead on point of aim at 50 yards, the bullet strikes 1 inch below the point of aim at 100 yards, 4 1/2 inches low at 150 yards and almost 10 inches low at 200 yards. If we assume that the typical whitetail buck measures 18 inches from brisket to backbone and its vital area measures roughly eight inches in diameter, it is easy to see that a hunter who shoots while aiming dead center of a buck’s lung area is okay out to 100 yards but his bullet actually lands outside the quick kill zone at 150 yards. A 200-yard shot with a dead center hold is out of the question for a 50-yard zero since the bullet lands almost 10 inches below the point of aim at that range. In order to place a bullet dead center of that buck, the hunter would need to hold the horizontal crosshair of his scope on the animal’s back.
The fellow who sights in his .30-30 dead on at 100 yards is better off because he can hold dead center on a deer’s lung area and his bullet will land in the vitals out to about 160 yards.
Now look at the fellow who zeroed his .30-30 three inches high at 100 yards. He can hold dead center and place his bullet into the vital area anywhere from a few feet off the toes of his boots out to about 225 yards. He simply holds dead center and squeezes the trigger.
That fellow has extended the effective range of his rifle about 65 yards farther than the fellow with the 100-yard zero and almost 100 yards farther than the fellow who sights his rifle dead on at 50 yards.
All of this applies even more to faster cartridges. Zeroing rifles in .30/06 and .300 Winchester Magnum three inches high at 100 yards rather than dead on at 100 yards, the point-blank range is extended by almost 100 yards.
Use A Solid Rest
The butt stock and forearm of a rifle should rest on something firm and yet resilient while it is being sighted in. Never rest the stock of your rifle directly on a hard surface and do not allow the barrel to m
ake contact with the front support. Either scenario will likely cause the rifle to shoot high.
Leather bags made specifically for use when shooting a rifle are available from a variety of shooting supply companies. After the bags are filled with sand, one is placed beneath the forearm of a rifle and the other rests beneath its butt stock. It is best to attach the front bag to an adjustable rest, but you can also rest it atop a couple of short lengths of wood.
If you do not have access to a sturdy bench rest, the bags can also be used while shooting from the prone position. But, you might have to elevate the front bag by placing it atop a few wooden boards.
In a pinch, you can cut 12-inch sections from a worn-out pair of jeans, sew one end closed, fill your homemade bag with dried corn or pinto beans and then sew the other end shut.
Close Range Zero Check
Any time I travel some distance to a hunt I always make it a point to check the zero of my rifle as soon as possible after I arrive. This is especially important when traveling by commercial airliner because no matter how sturdy a container a firearm is shipped in, it can still take a beating during baggage handling.
Strange things can and sometimes do happen to a rifle, so taking the time to check its zero is never a bad idea. It is also a good idea to know how to check zero when 100 yards is not available. Here is how I do it.
After sighting in a rifle to hit three inches high at 100 yards, I shoot a five-shot group while holding dead center on a target placed 25 yards away. I then write down the distance between the center of the group and the center of the target and file that information in my wallet. I can then check the long-range zero of my rifle by shooting a target at 25 yards.