The Shifting Zero
September 23, 2010
Here's how to sight in for success-- and it's not from the bench.
Snug the ring screws, bore-sight, then bench your rifle and fire and adjust until your ministrations plant the bullets where you're looking. You're zeroed, but you're not finished. In fact, if you take that rifle from the bench to the mountain, you may be setting yourself up to miss.
Zero is supposed to be a permanent bond between your line of sight and the bullet path. Truth is, that intersection can move--and vanish altogether--depending on how you hold the rifle.
In hunting camp and as a guide, I've often heard riflemen bellyache about "knocking the sight" off zero, or a "shifting zero." Always the whining starts after a bungled shot.
While loose mounts or a hard bump can change zero, and moisture can cause wood stocks to move, affecting the barrel's direction, such problems are less than common. The overwhelming majority of bad shots result from bad shooting. Of the few attributable to other gremlins, I suspect most culpable are changes in rifle support.
"Keep your position between shots," Earl told me. "Any change in position means a new landing zone for the bullet."
Shooting prone with a tight sling, van Zwoll typically sees seven o'clock hits, relative to bench groups.
My first smallbore coach had it right. When, firing prone, I moved my forward elbow to reload, the next bullet would follow a new path. The reason: Muscles impose different levels of tension along different vectors, and bones holding the rifle against gravity bear the weight from different angles.
If sling tension changes--or the angle or pressure of your shoulder at the butt, or your cheek placement on the comb--you've introduced additional variables. All affect the way the rifle reacts to a shot.
A perfect sight picture doesn't stay perfect as the trigger breaks. If you move the rifle while you squeeze, you can still hit consistently, provided the pressures at work remain the same, shot to shot.
The rifle always moves after the sear release. Striker fall and the trigger's contact with the stop induce some rifle movement or vibration. Ignition and the bullet's travel do, too. Your job in holding a rifle is not to keep it motionless--that's impossible--but to keep movements small and consistent.
A bench and sandbags or a solid commercial rest help reduce rifle movement, so when zeroing you can all but eliminate human movement from the sight picture. That's all in keeping with your mission: to engineer a collision of the bullet's arc with your line of sight at some specific distance. Reducing movement shrinks groups so you can adjust the scope more quickly and precisely.
The Savage .22-250 grouped tightly no matter the rest, but a block under the barrel pushed the group high.
But there's a hitch: Only in benchrest competition can you employ such an effective support after zeroing. Whether you're trying to finish a quarter-inch group in a 50-meter prone target or kill a deer at 300 yards, you must know where the bullet will strike from a position-supported rifle.
If you're a hunter, your rifle may not show slight differences in point of impact caused by a slight shift in elbow placement. But it can certainly throw bullets to a new place when you move from sandbag to sitting or offhand--or even if you change the contact point of the rest.
Some years ago, after zeroing a Ruger No. 1B from sandbags, I attached a sling to the fore-end and went prone. The group landed nine inches to seven o'clock at 200 yards. Now, a taut sling routinely pulls shots low. That's because the rifle is held down as the firing sequence makes the barrel shudder. The sling also pulls the fore-end away from the barrel, reducing pressure that would otherwise push the barrel up.
This .308 hit an inch higher with hand pressure on a rested rifle than one rested without hand pressure.
A sling tugging on a barrel stud can produce more dramatic effect, as it directly influences barrel vibration and can even bow the barrel.
Offhand without a sling there's no pull but no solid support either. So the rifle can dip as you press the trigger. As the trigger breaks, tiring muscles can involuntarily give way. Typically, offhand shots land low.
Rifle support can affect accuracy, too, and not just because some forms of support allow more rifle movement than others. Resting a rifle on a rock sets up different vibrations upon firing than does a rest that cushions the rifle.
Resting it against the side of a tree bounces it horizontally instead of vertically and can thus change the shape as well as the size of your group. To minimize changes in group placement, size and shape, it makes sense to pad the fore-end with your hand, always in the same place.
While 40 years of shooting have verified coach Earl Wickman's observations on position, only recently did I document changes in bullet strike resulting from changes in rifle support.
I chose three rifles and a rifled shotgun, punching five-shot groups from four positions: a commercial rest under the fore-end, gripping the stock's wrist only; a commercial rest under the fore-end, gripping wrist and fore-end; a padded block under the barrel; and a tight sling in prone position.
The chart shows the results (the rifles were fired at 100 yards, shotgun at 75). Numbers show changes in group placement (zero) and group size (group) in inches.
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A barrel-mounted swivel stud on a Ruger No. 1 in .375 contributes to low groups from a sling-assisted sitting position.
The .375 was fired only from a traditional rest and with a tight sling, for two reasons: First, with open sights I couldn't be guaranteed the tight groups necessary to detect slight differences in accuracy and point of impact after changing only my grip.
Second, a .375 is unlikely to be fired with a hard rest under the barrel. The sling test was meaningful because this Ruger has a barrel-mounted swivel stud. I suspected its effect would be greater than that of a stud affixed to the fore-end.
The average depression from zero with a taut sling was three inches at 100 yards for the three sporter-weight rifles. Elevation boost from direct barrel pressure for these rifles tested averaged 1.6 inches.
The tightest group from the .22-250 came with the barrel rested. For this rifle, consistent contact apparently trumped no contact. Pulling the fore-end down onto the rest was as likely to bump a shot high as to depress point of impact--maybe for the same reason a basketball dribbled hard next to the floor bounces higher than does one dribbled gently.
Group sizes varied little, even with marked change in point of impact. The vertical stringing I had expected from the barrel rest did not happen. Indeed, changes in support did not alter group shape as much as I'd anticipated.
Variables that affect bullet response to changes in rifle support include barrel weight and bedding, ambient temperature, stock design and material, lock time, sling placement and tension, rest placement and firmness, wrist pressure from your hand, cheek pressure and, well, the list is long.
Conclusion? The only way to find out where your rifle is shooting from any position is to fire a group or two from that position. Fire at least five per group. Repeat with a clean bore if you're a hunter, because you'll likely be firing the most important shot from a clean bore. Shoot from the positions you'll likely use on the hunt until you feel confident that the bullet will land at point of aim if you do your part.
I can hear the whining already, "I'm not going to waste all that ammunition!" Besides, some say, if the rifle shoots to different places from the bench and from hunting positions, what are you going to do? Change the zero you got at the bench? Well, yes, you are--if you want to hit where you aim in the field.
The Ruger No. 1 that drove its bullets nine inches to seven o'clock at 200 yards from a tight sling got a bedding job. I considered the difference excessive, so I relieved fore-end pressure by installing a rubber gasket around the fore-end screw.
Cinching the screw, I squashed that ring before pressing the wood against the barrel at the fore-end tip. My ministrations moved the low group up, halving the distance between prone and bench groups. Then I adjusted the scope to give me a 200-yard zero from prone.
I often zero from prone now. There's no sense firing from sandbags a rifle that earns its keep with a taut sling. Besides, zeroing prone is good field practice. I've watched many sportsmen check zero at the bench or over a pickup hood at hunting camps; I've seen very few take even one shot from a field position.
"It's an ego thing," said a hunting partner recently. "Shooters want to see tight groups, especially if they're checking zero in front of their peers before a hunt. They're supposed to arrive with a rifle already zeroed, and they're supposed to be competent marksmen. Shooting with the rifle unsupported, they'll likely push at least one shot so far out the audience will erupt in hoots and howls. Nobody wants that."
I think my amigo is right. Sometimes I've had to cajole hunters to fire a couple of shots before a hunt, even from a rest. When you don't shoot, you can't miss. But you delude yourself believing others will take you at your word when you say: "I've been shooting that rifle for 20 years. Punches sub-minute groups all day long to 500 yards. Never had to adjust the scope. It's a magnum, so wind doesn't bother the bullet."
Not long ago, I picked up a new rifle on my way to a hunt in the Far North. The 40X Remington hadn't arrived soon enough for me to wring it out at home first, so at camp I bore-sighted it and fired a round at a target taped to a big cardboard box at 100 yards. The bullet struck several inches off center.
After a half-dozen shots, all from prone with no support save my sling, the 180-grain Core-Lokt Ultras had moved to within a couple of inches of center, at 12 o'clock.
I could have hoarded the remaining rounds; instead, I kept shooting. The resulting groups gave me confidence in the rifle's cycling, as well as in its accuracy. Every shot helped me shoot the next.
When at last I got up from my zeroing session, I'd fired 15 of my 20 rounds, but after an hour on my belly on the tundra, I had a very good idea of where this rifle would send that bullet, what the reticle would do against the tension of my sling and how the trigger would feel as I crushed the last ounce from it.
With the next four cartridges I killed a musk ox, a caribou and a black bear, all from prone. There was no need for a second shot at the musk ox, but he did not want to expire; a finisher was only humane.
Shooting from hunting positions, and shooting often, will give you the confidence to shoot well in the field. That's much more useful confidence than the assurance that your rifle will print bottle-cap groups under ideal conditions from a bench rest you left 2,000 miles behind.