One-Third Second Rule – Rifle Zero

Is there one method that will work for sighting in all big game rifles?

In his book, Modern Hunting Optics John Barsness discusses how the 25-yard zero method used to apply and might still in some cases. But he also explains how it can get you in trouble. I’m sure folks have also heard of the point-blank zero method, along with all the other, so many inches high at 100, sight in options. If properly applied to the rifle, cartridge and load your shooting, they all work. However, properly applying the method, whichever it might be is the key. After spending about a week with a ballistics program and a calculator, and another week on the range for confirmation, I came up with a solution for sighting in. It’s called the One-Third Second Rule. It is the only zero method I know that provides the proper sight in for your maximum practical range (MPR) that will work for any big game rifle/cartridge/load combination.

MPR and Exceptions

Before describing this process let me make two things clear. First, let me explain MPR. It’s the distance where your rifle/cartridge/load combination can keep all of its bullets within a kill zone (six-inch circle) without worry about minimal wind drift or the ability of the bullet to upset/deform when it reaches the target. Secondly, there are times when you need a specialty zero. For example, when hunting here in the hills of West Virginia, I zero all my rifles at 100 yards because shots are generally somewhere between 30 and 130 yards.

Gravity and Time

External ballistics can be calculated because there is always a constant: gravity. Gravity pulls the same on all bullets, but since some bullets go faster and have a higher BC, it takes longer for gravity to get them to the dirt. That means there’s another constant, and that is time. Over a given time, gravity works the same on every bullet. The difference is how far that bullet can travel during that time

The One Third Second Rule

Regardless of the bullet, its velocity, or BC, with the proper zero it’s possible to keep it within a six-inch impact zone out to the distance it can reach in one-third of a second. Also – most often – a full value wind of less than 10 mph will not push a bullet egregiously outside of that six-inch circle if it only acts on the bullet for one-third of a second. And finally, all big game bullets will still impact with enough velocity to induce optimal bullet upset all they way out to one-third of a second from the muzzle, regardless of caliber or construction. All you need to do to sight your rifle in with the One–Third Second Rule is determine how far the bullet will travel in a third of a second and then adjust your sights so the bullet will impact three inches below your point of aim at that distance. Your bullet will then cross your point of aim (POA) once, very near 25 yards, and again, at some distance short of the distance it will travel in one-third of a second. A ballistic program will give you all of this data but be mindful to input the exact distance that the center of your riflescope is mounted above the center of your bore. If you fail to do so, the ballistics program will give you bad info. (Barsness also explains this in his book in the chapter about the mythical 25 yard zero.) It will also give you bad info if you fail to input the exact muzzle velocity of your load. So, you will need a chronograph too.

An Example

Let me see if I can illustrate how this works. My favorite load for my New Ultra Light Arms Model 20 Short in .30 Remington AR is a 150 grain Nosler AccuBond at 2600 fps. That load has a MPR (one-third of s second range) of about 260 yards. This means the bullet will first cross my POA at 26 yards and again at 220 yards. It will reach its maximum height above my POA at 125 yards (3.00 inches) and its maximum distance below my POA at 260 yards. To zero, I can sight the load in three inches low at 260 yards, dead on at 220 yards, three inches high at 125 yards, 2.75 inches high at 100 yards and even dead on at 26 yards. (Keep in mind a ¼ inch mistake at 26 yards would equate to about a one inch mistake at 100 yards or a 2.5 inch mistake at 260 yards.) With a full value 10 mph wind, the bullet would drift six inches at 260 yards and I can simply nudge the reticle into the wind slightly to compensate. It would also strike the target with a velocity of almost 2100 fps which is plenty fast enough to make the bullet fully upset.

Another Way to Skin the Cat

To get a better idea of how this all plays out, the table below lists the MPR or one-third second distances for several cartridges. Obviously, these are generalized and your load would alter these figures slightly. The bottom line is if you use the One-Third Second Rule as a zeroing guide and never shoot beyond your MPR, you’ll never have to worry about where to hold or if your bullet will work when it gets there. You’ll also not need to worry too much about animal movement between the time you pull the trigger and when the bullet gets there. One-third of a second is not very long at all. It may not be the best method but it is a method that answers a lot of questions and works for any rifle, shooting any cartridge and any bullet. With the availability of chronographs and ballistic programs it’s also very easy to apply.