The only way of being sure to sight in your outfit properly is over a solid bench rest with a steady rifle rest.
A rifle is useless for hunting unless it is properly sighted in. Before you start sighting in, it is important to understand the pull that gravity exerts on a bullet in flight. A bullet fired from a horizontally held rifle over a level area, will strike the ground at the same time as a bullet held in the hand at the gun’s muzzle and dropped at the same instant. The first bullet will strike the gound several hundred metres away, while the second bullet will drop straight downwards. Both bullets will fall at an identical rate of speed. According to the formula, “distance fallen equals half the acceleration of gravity, multiplied by the time squared,” hence both bullets will fall 16 feet (4.877 metres) during the first second of free fall.
The path of a fired bullet is never flat, because it starts to drop as soon as it leaves the muzzle, and the downward curve is accentuated as velocity falls off. This path, called the trajectory, traces an unsymmetrical line called a ballistic curve. The second half of the curve does not match the first half because drag slows the bullet. The highest point is about 55 per cent of the curve when the sights are set so the bullet will land at a specific distance. And the total drop is four times the mid-range height based from the sight line.
These days the majority of shooters use a scope mounted on top of the rifle, parallel to the bore, and the centre of the scope is on average 38mm higher than the bore. Since the bullet drops down from the bore for every metre is travels laterally, its flight will never coincide with the line of sight at any distance, unless compensation is made. In other words, the rifle will not shoot where it looks.
The flight of the bullet curves, beginning below the line of sight, and since the target is usually set up at 100 metres or yards, the bullet must start under the line of sight in order to intersect it. It then rises upward, to become level with the line of sight, and continues above it, before finally curving downward, usually at a distance of 200 to 300 yards, when it again crosses the line of sight. As a rule, the second intersection of the bullet with line of sight will be the zero range for which the rifle is sighted in.
When sighting in, windage and elevation adjustments are made to the scope so that the line of sight dips down into the line of the bullets flight. In other words, if the scope is held horizontally, as they are for the majority of shots, the bore points slightly upward so that its line of flight will intersect the line of sight and hit the target.
There are several different ways to sight in a rifle, but nothing is better than a solid bench with some kind of rifle rest or sandbags sat on top. After mounting a scope, the best method involves first using a collimator to get the scope’s reticle aligned with the centre of the bore. These little instruments are cheap and every serious shooter should have one.
Once the bore and sights are made to coincide, the initial sighting and test firing is usually done at 25yds. Fire three trial shots to compensate for errors of aim and allow for any slight inaccuracies in the rifle. Next, draw lines between the three bullet holes to form a triangle. Then mark the exact centre of the triangle with a marking pen. This will indicate the mean point of impact. Now, make adjustments by turning the two dials on the scope until the rifle lands its bullets exactly on the point of aim at 25yds. The rifle is now sighted in at this distance.
With cartridges having a muzzle velocity of from 2800 to 3000fps velocity, the rifle will be zeroed at just over 200yds, a good range to sight in for most hunting purposes. For example, a .243 Winchester shooting 100gn factory ammo and sighted in to be dead-on at 25yds will normally land .75” high at 50yds, 2” high at 100, 2.75” high at 150, and 1.75” high at 200, 1/2” low at 250, 4” low at 300, and 9.75” low at 350. Other factory loads having the same velocity and shooting bullets with similar ballistic properties will have similar trajectories.
It is always wise to finish sighting in by firing several shots at 200yds. The 100gn .243 bullets should print about 1.75” high. If they do, leave well enough alone since it won’t matter a bit if the bullet lands a trifle high when you’re holding dead-centre on deer-size game.
One method of zeroing standard cartridges for those who are limited to a 100yd sighting in range is to find the mid-range trajectory height of a particular cartridge at 200yds and sight in so the bullets strike that high at 100yds. Then the rifle will be sighted in at 200yds. The bullet drop for longer ranges can be determined later from a ballistics table.
Another method is to simply adjust the scope that the bullet lands 3in high at 100yd. Most modern cartridges, sighted 3in high at 100yds, will land their bullets on point of aim at just over 225yds. The bullets will drop about 5 to 7in low at 300yds, depending on the shape, sectional density and other factors affecting sustained flight. This trajectory will allow the hunter to hold almost dead on for ranges up to 250yds or more and still score a solid hit, on deer-sized game. But he will have to learn the bullet drop for longer distances and how much holdover is needed.
Many big-game hunters prefer to sight in by zeroing their rifle (with the load they are using) to take advantage of their rifle’s maximum point blank range to gain the flattest possible trajectory. With the rifle perfectly sighted for that distance, they memorize the trajectory height for mid-range distances and hold accordingly. As a rule, the higher the velocity, the flatter the trajectory. This is particuarly true with larger-capacity small-bore magnum cartridges shooting ballistically efficient bullets.
For example, a hunter may elect never to shoot from over 400yds at the deer-sized game for which his .270 WSM is well suited. He may further decide to use only 130gn factory ammo with its initial velocity of 3275 fps. To achieve his goal, he extends his zero range to have the bullets strike the point of aim at 300yds. Referring to a trajectory table available in brochures, he finds that the trajectory height over that range peaks at 3.56”. When actually shooting at game, if the hunter finds the distance with his laser rangefinder measures 300yds, he holds dead-on, but even at 400yds where the bullet drops 8” a dead-centre hold on a deer will usually result in a heart shot. If the game is about half this distance, the hunter doesn’t have to hold a bit low with this flat shooting cartridge, because the bullet rises less than 3”. Even at 500, holding the crosswires in the scope about 12” above the game’s backline will land a bullet in the chest cavity. Incidently, way out yonder at 500yds, the remaining velocity of the 130gn Nosler Ballistic Tip .270 bullet is 2338 fps and energy 1578 ft/lbs – entirely adequate killing power for deer – as long as the bullet is well-placed.
This then, for all practical purposes, insures a clean hit on deer-sized game and is a good way to sight in for open country where long shots often present themselves.
This article was first published in Sporting Shooter, Aporil 2012
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