“Yes, that WILL change your zero!”

[Newsletter]

"Yes, that WILL change your zero!"
By: TimOren - Formerly known as Nero; August 25, 2013

Editor's Note: This month's marksmanship article is another great one from our archive.

Often, during the afternoon of a shoot, we'll see shooters' groups start to shrink -- great! -- but find they are not on the target -- not so great! -- even though the rifle was zeroed at 25 meters in the morning.  Punching out a nice 4 MOA group but getting a poor score for it can be frustrating.  What's going on?

There are a number of things that can change a rifle's zero, some of them well known to experienced shooters and others less known.  This article reviews many of them, starting from the obvious and working towards the more obscure:

* You changed ammo.  Given the ammo shortage, we're seeing Appleseed shooters who have boxes of more than one brand, particularly of 22LR.  If you change brands, your zero will likely change.  It's most obvious if the velocity (fps) rating of the two rounds differ, but even rounds with the same nominal velocity may differ in bullet weight or shape, or smaller details like rim thickness and primer strength.  If you must change ammo lots in mid-shoot, try to do it at the end of Saturday so you'll get a chance to re-zero on Sunday morning.  If worst comes to worst and you run short on Sunday, let the line boss know you'll be hanging a squares target so you can rezero, instead of an AQT.

* Something came loose on your rifle.  Check your scope rings and rail, or iron sight mounts, and the take-down screw on your rifle.  Appleseed shoots are notorious for beating up rifles.  Loosening screws might result in groups that just get bigger, but could also cause a zero shift if (for instance) a front sight slips in its dovetail, or a rail works loose from the action.

* You changed your scope's magnification.  If you've got a pricey scope, this should be OK.  But if you've got something less than a Leupold, it may make a difference.  Pick one setting at the beginning of the shoot and stick with it.  4x will do most people just fine.  Go above 6x and you may get seasick in standing (and it's a temptation to jerk off a shot).

* Many causes of zero change boil down to one thing:  cheek weld.  As you may have heard from an Appleseed instructor, cheek weld is sight alignment -- and sight alignment plus sight picture determine the zero.  Even subtle changes in cheek weld may tweak the sight alignment enough to make a difference:

* You changed your scope's eye relief.  Probably a good thing -- many scopes come from the shop mounted too far to the rear -- but you will definitely change your neck extension to work with the new scope position and that will change cheek weld.

* You changed the length of pull.  Another way to compensate for a too-close scope or too-short stock is to do a quicky build-up on the buttstock.  The stock is now in a different position relative to your head once you've done your turkey neck.

* You built up the stock comb.  Again, a good thing to do if you can't get a firm cheek weld.  But, pretty much by definition, it will change that cheek weld and therefore zero.

* You changed your position construction.   During a shoot, you'll be going through a combination of learning and improving steady hold factors, and adapting the position to your body build and rifle.  This is normal and expected, but it may result in subtle changes in cheek weld.  This means that the zero that worked in the morning may no longer be correct after a day of learning and adjustment.  This is often obvious in sitting -- most shooters new to the position go through a lot of tweaking before finding a way that works for them.

* You changed sling length / tension.  Many shooters tighten up their sling as they get used to it, and that often helps improve results.  But it does change the position construction, whether or not you think of it that way!

* You zeroed from a bench.  This is for the well-prepared who got their zero before coming to the shoot, but did it at a range where position shooting is not available.  Cheek weld in prone and at a bench are different enough to make a difference.  You will be close, but don't skip rezeroing on the squares!

* You have an electronic sight and are moving your head around.  No matter what the advertising guff for that fancy sight said, head position does matter, at least for precision rifle.  Adjust your head position so the dot or laser image is at its brightest and use that position consistently.  Turn the brightness down to the minimum that's reliably visible, to minimize 'bloom' that obscures your target.

* You changed shooting eye.  You might be dealing with eye dominance issues, or just borrowing a rifle that was sighted by someone who uses the opposite eye.  My wife and I shoot using opposite eyes, and we regularly see a 6 MOA difference in windage when we trade off our favorite rifle.

If you put up a group that's off the target a few times, and in various directions, that's probably an NPOA issue.  But if you start seeing the groups consistently away from your point of aim, your zero might have changed.  Run through this list and see if any of them apply.

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License by Tim Oren.