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Shooting Tips: The 1/3-Second Zero

Let a bullet's time of flight determine how to sight in a big-game rifle.

Shooting Tips: The 1/3-Second Zero

Photo by Richard Mann

There are many ways to sight in a rifle for hunting big-game. Adjusting a scope’s reticle to cross the bullet’s path of flight is only one aspect of the process. Selecting the most practical point along that path where the reticle intersects—in other words, the distance at which the scope is zeroed—is another. After nearly half a century of trying just about every method, I’ve settled on what I call the 1/3-Second Zero. I’ve come to realize it offers the most pragmatic approach.

Before I explain the process, you need to understand something about external ballistics and trajectory. Regardless of the cartridge or bullet, the amount of bullet drop over a specified duration of time is a given. This is because the effect of gravity is a constant; it pulls on all bullets the same. In the span of 1/3 second, bullets fired from a .30-30 Win. and a .30-06 Sprg., for example, will drop the exact same amount. This may seem hard to believe since everyone knows a .30-06 shoots much flatter than a .30-30. But we’re talking about time, not distance. In that 1/3 second the .30-06 bullet will travel farther, but it will drop the same amount as the .30-30 bullet.

Why is 1/3 second important? The amount of drop during 1/3 second of flight allows us to adjust the reticle so the bullet will never strike more than 3 inches above or below the line of sight out to the distance the bullet can travel in that amount of time. With the proper zero, you can hold dead-on out to the 1/3-second distance and the bullet will impact within a 6-inch circle, which is well within the vital zone of most big-game animals.

The only way to know a load’s true 1/3-second distance is to measure its muzzle velocity from your rifle with a chronograph. Then apply that velocity to a reliable ballistics calculator and find the distance the bullet will travel in 1/3 second. That distance becomes your maximum practical range, or in better words, the range at which you can hold dead-on and hit a big-game animal’s vitals.

Use the ballistic calculator and adjust the zero until the bullet strikes 3 inches low at the 1/3-second distance. Keep that zero and check the drop table to find out how high the bullet will strike above point of aim at 100 yards. Sight in your rifle at 100 yards so the bullet strikes that high. Finally, confirm the zero and fine-tune your adjustments if necessary.

Once you have zeroed your rifle to get maximum benefit from its 1/3-second distance, you can hold dead-on to that distance without worrying about trajectory. This system eliminates guesswork out to a range most would consider practical for the given cartridge. You will not have to hold over, click in a correction or use a bullet-drop reticle to get a hit in the vital zone. It saves time in the field and allows you to focus on taking the shot as opposed to adjusting for it.

Shallow Trout

No Holdover Required

With the proper zero, a rifle will place a given bullet within a big-game animal’s vital zone from point-blank range out to the distance that bullet travels in 1/3 second. The distances above are estimates based on typical hunting loads; actual distances will vary with differences in bullet velocity and ballistic coefficient.

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