## Fastest Round Average Velocity Slowest Round

the perfectly lazed range to the precise compensation you applied. But you sdll missed.

The reason was the change in altitude. You had zeroed your rifle at the Kabul Airport, elevation 5,931 feet above sea level, and attempted to engage the al Qaeda fighter on a 15,000-foot ridgeline, some 9,000 feet higher. The problem is not chat gravity decreases when you're farther from the center of the earth; it's that the air becomes thinner and there's less drag, causing your bullet to fly faster with a flatter trajectory.

It has long been known that jet fighters attain their greatest speeds at high altitude because they zip easier through thinner air, but few riflemen realize this principle applies to their bullets, too. And if the altitude is high enough, it will throw your Bullet Drop Compensator completely out of synch with your trajectory, no matter if you rezero, because the BDC is designed for the M118LR bullet's sea level trajectory.

There are special ballistic formulas to compute these effects, but having looked at these three-step algebraic beasts—which change slightly according to humidity, barometric pressure, and temperature—I've concluded that the only practical solution is to forget about computations and just plain rezero your rifle, then test it at 100 yard/meter distances to see how dose the BDC is to true.

Although it's not truly precise, there is a general rule of thumb when compensating for a change in altitude: you should add or subtract 1 MOA for each 5,000 feet of elevation change. But this will vary with the distance you're shooting. Army tests have shown, according to Hatcher's Notebook (a shooting ballistics bible), that Ml boat-tail bullets fired through the Garand rifle required 8 MOA additional elevation when fired at sea level, compared with those fired at 10,000 feet, for a range of 1,000 yards.

If at all possible, you should rezero for the altitude at which you'll operate and test-fire at assorted distances to fine-tune your BDC or target knob settings.

TEMPERATURE AND BULLET TRAJECTORY

The rate at which gunpowder burns varies with temperature. The colder it is, the slower it's consumed and the less velocity it imparts on a bullet.

This velocity, in turn, determines your bullet's trajectory, and here's where it gets important. When the temperature changes enough—getting your powder either hotter or cooler than when you zeroed your rifle—the bullet will depart from zero and impact higher or lower than your point of aim.

We've been able to compute these effects by crunching several kinds of data to show you how significantly it can affect your shooting. This data is not ironclad or perfect, but it's very close. Note that 59°F (1 S°C) is the industry standard when computing ballistics.

At 300 yards or less, the differences are worth noticing but not very dramatic. At 500 and 600 yards, however, trajectory changes can determine hits and misses, and these temperature swings can occur in a single day in the desert. I recall 59-degree dawns and 100-degree noons in the Mojave. Mountain temperatures may vary as much as 25 degrees in a day, too.

For several years I've wrestled with various ways to calculate compensation for a change in tempera-

ture—including some dating back to the 19th century—and found most rules of thumb too general to be called precise. The sole exception, which I've tested and verified to be pretty consistent, is the following, which best applies to a .308 round:

• When the temperature changes 20 degrees, apply 1 MOA at 300 yards.

• When the temperature changes 15 degrees, apply 1 MOA at 600 yards.

When the temperature changes 10 degrees, apply 1 MOA at 1,000 yards.

Obviously, if it's an increase in temperature, your bullet's flying faster/flatter so you lower your sight; if the temperature fails, your bullet travels slower, so you have to raise your sight. The logic behind this method is that the temperature change will first be felt at the greatest distance— because that's where even a slight change will matter. By contrast, it will take quite a temperature change to matter at only 300 yards. If you study the adjacent table titled "Temperature Effects on Trajectory," you'll find that its data is consistent with this technique.

We recommend that you reconfirm your zero when there's a temperature change of 20 degrees or more. And when operating in an area that experiences wide daily temperature fluctuations, you must test-fire, record results, and apply them to your shooting. To facilitate this, make an inexpensive clip-on chermometer, available at backpacking stores, a part of your basic equipment.

Ha\ing been convinced that cartridges perform differently at different temperatures, you should agree that a new challenge arises: keeping the cartridge/powder temperature consistent from round to round. Therefore, in winter make sure you don't carry any reload ammo close enough to your body that it gets warm, and don't chamber a follow-up round until the chamber has cooled.

Likewise, in summer don't fire rounds that have been lying in direct sunlight until they've cooled. The closer you can keep your ammo to the temperature it was when you zeroed your rifle, the fewer complications you'll have.