How to Calculate Bullet Drift

A deer hunter with his prize

Bullet drift, also called wind drift, is one of the major causes of missing a long-range target. Along with bad shooting technique or a misaligned scope, it's one of the most common problems.

A good scope comes with a tool that allows a shooter to adjust for wind factors, but that is effective only if the shooter knows how far to make that adjustment.

Because calculating bullet drift requires some complicated math, a shooter should compute typical drifts in advance so those calculations need not be done under pressure in the field.

Things You'll Need

• Kestrel or other wind-speed reader (optional)

Instructions

1. Determine the wind speed. A kestrel or some other device that reads wind speed would help eliminate guesswork, but shooters have been estimating windage without electronics for generations. A handy but simple rule of thumb: A breeze you can feel on your face is 2 to 4 mph, one that moves tree leaves and branches is 5 to 9 mph. Wind starts raising dust at 10 mph, and wind that sways trees is 12 mph or higher.

2. Determine the wind direction. This can be as simple as licking your finger and sticking it in the air if it is not already obvious.

3. Combine this information with the caliber and weight of your bullet and its muzzle velocity to form a bullet-drift compensation value. Ballistics tables for a given ammunition can help with this, as the numbers will be different for every kind of ammunition. Common hunting round is the 175-grain, .308 round--also known as the 7.62mm NATO round. At a muzzle velocity of 2,600 fps and 10 mph of wind, this round has a compensation of 3 inches at 200 yards, 7 inches at 300 yards and 22 inches at 500 yards.

4. Decide what the angle is between your line of fire and the wind to determine how much further you will need to compensate for bullet drift. If the wind is blowing directly off your right or left--a 90-degree angle--use the full value found in Step 3. Reduce the value for sharper angles: 60 degrees=0.85, 45 degrees=0.7, 30 degrees=0.5. A wind running directly in front or behind you has a value of zero, and no compensation is necessary. Use the compensation value to determine how far into the wind that the aim must be compensated. If the 10 mph wind were blowing at a 30-degree angle from the right of the shooter with the .308 rifle and the target is 300 yards away, the aim must be 3.5 inches to the right of the target.

Tips & Warnings

• Many telescopic scopes have bullet drift/windage compensators that add some precision to the process of aiming into the wind. Muzzle velocities can vary among different models of firearms, and not all ammunition of the same caliber have bullets of the same weight. This can result in slight differences that count in long-range shooting, so it pays to do a little research on your own rifle and to alter your numbers whenever you switch ammunition.

References

• Ultimate Sniper Windage Guide
• Reading the Wind
• Determining Wind Values