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.