THE PERFECT ZERO?

 By

Craig Boddington

Previously, this column discussed the process of “sighting in.” If you’re happy, then we’re done; it’s time to head for the deer stand! We’re going to assume we have enough accuracy to reliably hit a deer’s vital zone at whatever distance we might shoot. The vital zone of even a small deer offers about an eight-inch target, so extreme accuracy isn’t essential for much for field shooting.

Hey, I love tiny groups because they instill confidence, and I love to ring steel at long range. However, I’m unlikely to shoot at a game animal much past 400 yards. Most of my shots at game are much closer, and many of us rarely need to reach past 200 yards. Theoretically, if your rifle is producing one-inch groups at 100 yards (one Minute of Angle or “MOA”), then it should produce two-inch groups at 200 yards, four-inch groups at 400 yards, and so on. Considering the size of the vital zone, one MOA is more accuracy than essential.

Actual groups usually get larger as distance increases, so I don’t mind having more accuracy than I really need, but let’s be reasonable and practical. Even today, with the best rifles, optics, and ammo ever, not all rifles can produce one MOA accuracy.

Not a train smash; 1.5 MOA is plenty for most field shooting. Most modern rifles will do at least this well, and that’s “good enough,” at least at normal field ranges. I have older rifles that are “two MOA” rifles.  Also not a problem. I hunt with them, but only in close-range situations! With such rifles, I usually do my zeroing on ten-inch paper plates. In that context, “paperplate accuracy” is good enough! Regardless of the accuracy you have to work with, and the ranges you might consider shooting, you still must decide exactly where to leave your rifle zeroed before you head afield.

TRAJECTORY CURVE

Traditionally, most of us leave a rifle zeroed slightly high at 100 yards, to take advantage of the bullet’s trajectory. Here’s how this works: There are two straight lines, line of bore and, slightly above, line of sight. Both are straight, but the path of the projectile is curved. Gravity starts working on any projectile as it leaves the muzzle, and air resistance slows it down. As distance increases, the projectile falls ever more quickly, eventually striking the ground.

If line of bore and line of sight remain parallel, the bullet will never cross the line of sight and no zero can be achieved. Using sight adjustments, we actually zero so the line of bore and line of sight slightly converge. Line of bore remains straight, while the projectile’s path is curved. With line of bore tilted slightly upward relative to line of sight, the projectile’s curving path crosses line of sight twice, once at short range and again farther out. In between these points the projectile’s path will be above the line of sight. The point at which this distance above line of sight is greatest is referred to as “mid-range trajectory.”

The steepness of the trajectory curve depends on velocity and projectile aerodynamics. In establishing final zero, we usually try to use that curve to best advantage, extending the ranges at which we can shoot without having to worry about holding off the target (above or below) to compensate for that curving trajectory.

There should be little mystery about the actual trajectory curve. For generations, printed ballistics charts have yielded this information, usually suggesting various sight-ins at 100 yards (the bullet’s first crossing of line of sight), and telling us greatest height of trajectory, and where the dropping projectile crosses line of sight again, and yielding bullet drop at various ranges as the decline accelerates.

Today, ballistics programs and smartphone apps yield the same information, and allow us to input altitude, temperature, humidity, barometric pressure, and more, all of which increase in importance as range increases. Printed data assumes a standard measurement of line of sight over line of bore (height of scope). Electronic data allows us to input this. With the larger (higher-mounted) scopes in vogue today, that measurement must be accurate.

All data, whether printed or electronic, assumes that the starting velocity is correct. Barrels vary in length, and there are “fast” barrels and “slow” barrels. For truly accurate data, it’s essential to use a chronograph to check the speed of your load in your rifle.

Fortunately, the vital zone of a big-game animal remains a large target! None of this stuff matters much if your goal is to shoot your buck from a favorite treestand, like in thick timber at my Kansas farm. When I’m setting up a rifle for an open-country hunt, you bet I measure height of scope and check velocity! Effects of altitude and climatic factors are less critical…until you get past normal shooting distance, or you have extreme variations. In preparation for fall hunts, I do my summer shooting in hot, low country. I make a guess on anticipated altitude and climatic factors, run the data, and zero accordingly. This has proven adequate for the ranges I shoot at game…but isn’t precise enough for extreme-range work!

DEAD-ON OR SLIGHTLY HIGH?

Whether at 25, 50, or 100 yards, a dead-on zero with a modern rifle cartridge is the first time the bullet crosses the line of sight. Farther on, it will be above the line of sight and, as the curve steepens, it will cross line of sight again somewhere downrange.

It is not true that “dead-on at 25 yards” will be close to “on” at 100 yards. This is possible with slower cartridges, and with iron sights or low-mounted scopes. With faster cartridges and higher-mounted scopes, I’ve found that a 25-yard zero will usually strike too high at 100 yards. A 50-yard zero comes closer, especially with low-mounted sights. I often zero iron-sighted rifles and scoped big-bores at 50 yards and call it done, knowing that I’m unlikely to use such rifles much past 100 yards. However, with today’s big scopes, I find that a 50-yard zero is usually three or four inches high at 100 yards. This puts the second crossing of the line of sight ‘way out there, and creates a mid-range trajectory as much as six inches above line of sight. For me, this increases risk of shooting over an animal (or hitting too high).

For close-range work, there’s nothing wrong with a 100-yard zero. Depending on cartridge, “dead-on at 100” will be on again at 150 to 175 yards, with little mid-range-rise. More common is to zero a “couple of inches high” at 100 yards. You can study ballistics charts and programs, and you should. Depending on your cartridge (and load, and bullet), a zero of two to 2.5 inches high at 100 yards will put you dead-on somewhere between 200 and 300 yards. You shouldn’t have to hold low at closer range, and you shouldn’t have to hold over until nearly 250 yards. In my youth, Jack O’Connor was our greatest gunwriter. His consistent advice was to zero “two to 2.5 inches high” at 100 yards. I believe his formula remains sound, and that’s the way I usually zero for general-purpose use. Most important to me: I never establish a 100-yard zero any higher than that, because of the risk of shooting over at “medium” range!

DIALING THE RANGE

These days, dial-up turrets are all the rage, and they change the game. Some systems require either a 100 or 200-yard zero as the starting point. If you intend to dial the range, then I assume you may be shooting at some distance. I don’t like a 100-yard zero in open country, simply because you must start holding over (or dialing) at fairly close range. With today’s optics, dialing is precise, but fraught with human error: You must dial correctly and, if you don’t shoot, you must remember to dial back to zero. (Trust me, everybody forgets now and then!)

I’ve used several systems with good results, but a favorite is Leupold’s Custom Dial System (CDS), with turret calibrated to my load at a stated altitude and temperature. On these, again, I strike an average of most likely conditions. My CDS is based on a 200-yard zero. At 250 yards I’ll usually hold slightly high on the shoulder, keeping it simple and taking advantage of that large vital zone. I normally don’t consider dialing until about 300 yards.

If your system is based on a 200-yard zero, then you should check zero at the actual distance, so your starting point is verified as correct. Then, if you’re serious about shooting at longer ranges, you need to verify your data all the way out. This is a stumbling block for many who don’t have ready access to a “long” range. Sorry, whether published or electronically generated, data cannot be considered valid until verified by shooting at actual distance. The farther you might consider shooting at game, the more critical this becomes!

Finally, if you’ve traveled some distance—by any means—it’s important to check zero when you arrive at your hunting destination. There’s no consistency about how much (or how little) rattling around may cause a shift in point of impact, so it’s always worth checking. On long, tough hunts, I’ll usually check zero every few days, for sure if the rifle has been dropped! I also recommend checking zero after an inexplicable miss. It’s terrible for the ego, but great for peace of mind to know for sure it was your fault! When planning ammo for a distant hunt, factor in enough to check zero about three times!