What causes scope shift?

By B.B. Pelletier

Scope shift is a term that describes the movement of a gun’s zero over time. While the gun remains very accurate, the point of impact changes location so you don’t know where the first shot will go. That’s frustrating for hunters and anyone who just wants to shoot and not fiddle with their scope adjustments. Lots of things can cause this, including:

• Shooting at different ranges
• Cant
• Loose scope mounts
• Loose erector tube inside the scope
• Hold (where the sighting eye is placed)
• Scope power adjustment
• Sloppy shooting technique

This is a long list, so we’ll tackle some of these issues in another message. Let’s start with shooting at different ranges. The pellet’s location will go up and down as the range changes, but, when we sight in, we try to pick the pellet’s most likely spread of ranges at the point of aim. Learn more about this in the sighting in a scope article.

Do your groups move to the right? Or left?
Unless your scope is optically centered, the aim point can easily drift right to left (or vice-versa) as distances change. At 10 yards the pellets may land an inch below the aim point – which you would expect – and a half-inch to the right – which you wouldn’t. Without changing the scope’s zero, the pellets may be dead-on at 25 yards and an inch to the left, as well as the expected inch low at 40 yards!

That right-to-left movement of the groups IS NOT scope shift, although many shooters think their scopes are to blame. To correct this problem, use adjustable scope mounts and optically center your scope. B-Square is the top producer of adjustable scope mounts and has many different models of their AA mounts for different air rifles. Optical centering needs a whole message of its own, so I will save it for later.

What about cant?
Cant is the unintentional tilting of the rifle when shooting. If the rifle is zeroed while level, but then canted to the right, your shots will land low and to the right. At 50 yards your group could move three inches to either side of the center point!

To eliminate cant, mount a scope level on your rifle and center the bubble before you take each shot. I recommend the B-Square bubble level that sticks out to the side. It may look odd, but it’s the only one you can see while sighting through the scope. Put it far enough in front of your eye so you can see the bubble with your non-sighting eye.

I’ll review each of the other causes for so-called scope shift in a later message. Until then, keep ‘em in the ten-ring!

15 Responses to “What causes scope shift?”

  • Matt Says:

    Hi B.B.,

    After discussing with you potential problems for my reduced accuracy at closer range, I did go back and concentrate to “aim small.” Sure enough, my groups at 10 yards shrank to 1/4″.

    HOWEVER, I’ve got a perplexing new problem. About 20 to 25 shots into enjoying the new results (1/4″ groups at 10 yards, CFX, Leapers Bug Buster 2, non-adjustable mounts), a series of about 6 shots began to creep down from the bullseye about a 1/2″, and about 1/4″ left. I could see this happen, because I was shooting one shot at each bullseye on a sheet of about 20 targets.

    This was a LARGE change, and as I continued aiming for the centers as I allways had, the following shots settled right in to 1/4″ groups just like before, but in this new spot 1/2″ down and 1/4″ to the left of the bullseye.

    This new spot resulted when I tried again the next day. Same accuracy and consistency, just a shifted strike zone. I am CERTAIN that all along I have had the same hold, same rest, same concentration, same ammo, same range, etc.

    Any ideas?

    Matt

  • B.B. Pelletier Says:

    Matt,

    No ideas as long as the scope adjustments are in the center of their range. Stuff happens, I guess. Did the temperature shift when this happened? That will cause an aimpoint shift.

    B.B.

  • Matt Says:

    Nope, same temp – the shift happened literally in the course of 6 shots (4 minutes?). I’m befuddled myself. I’ll double check the mount screws – otherwise, I guess I’ll just rezero…

    Matt

  • Anonymous Says:

    hello sir,

    its my first time to zero.on top of the knobs there is the arrow point to the right, but when i turn to the direction as the arrow is pointing the recticle goes left instead of going right, the same with the elevation when i turn down it goes up instead and vice versa.something wrong with the scope?

  • B.B. Pelletier Says:

    Some Chinese scopes use German diopter sight markeings, which are the reverse of the norm. Thye German sights tell you “bei links” and “bei rechts,” which doesn’t translate well literally, but you can think of them as “too left” and “too right.”

    Just memorize that and the scope ajustments will seem normal.

    B.B.

  • Anonymous Says:

    follow up question sir.i am refering to BSA4x34AO air scope. i think it’s manufactured in China although it has a US warranty.how would you rate this scope? i am using a CO2 rifle.what ranges should i zero it?

    thank you.

  • B.B. Pelletier Says:

    I don’t care for BSA optics. Gamo, who owns BSA and BSA Optics, sources the cheapest scopes they can. Nearly all scopes are made in China these days, but Bushnell, Bausch & Lomb, Air Force and Leapers all put some money into their products. In my experience, BSA scopes have been dark and murky.

    That said, you may have a very good one. If it serves your purpose, what I said means nothing.

    I would zero my scope for 20 yards. That is an ideal airgun zero distance and your flat spot will extend out to 27-35 yards, depending on the velocity of your gun.

    B.B.

  • Anonymous Says:

    B.B.-

    I have a problem with something similar to scope shift, only I am using iron sights. When shooting, my Gamo Multishot, after about 8 shots, my elevation adjustment for my sights turns clockwise 2 places, in turn, forcing each shot slightly lower than the next. If I do not, after each shot, move the adjustment knob to where it was previously, I will eventually be shooting into the dirt. I have watched this happen and I know I am definitely not moving the adjustment knob myself. I do not know what explains this and I do not know how to fix it/prevent it from happening. If you have any help, I would really appreciate it.
    Thanks,
    Tim

  • B.B. Pelletier Says:

    Tim,

    That’s a new one! I would guess that your rear sight is adjusted all the way up; am I right?

    You might try some Locktite on the screw of the adjustment wheel. Be sure to clean the threads and only use blue number 242 Threadblocker. The screw will be stiff but movable after the Locktite is applied.

    B.B.

  • Anonymous Says:

    B.B.-

    You’re right, my rear sight is adjusted almost all the way up. I will try what you suggested. Thanks so much!
    Tim

  • andreas Says:

    B.B.

    How does scope power affect the impact point?

    I just had the scope for one day and I didn’t have the time today to test it for myself…

    Thanks a lot

  • B.B. Pelletier Says:

    andreas,

    Test it. Some scopes shift with power changes. Others don’t.

    B.B.

  • andreas Says:

    Thanks B.B.

    I will test this as soon as I find out why the rectile is blur when I am at x14 and perfectly sharp when I am at x4.

    The target is sharp – or at tleast my eyes make it sharp. I ‘ve tried abjusting the eyebell but nothing helpful happened. The AO is at the same distance for both settings. – should I change it?

    Why do you think this happens?

    Thanks a lot

  • B.B. Pelletier Says:

    Andreas,

    Only adjust the eyepiece when looking at a light surface, such as a wall or the sky.

    Then focus the scope with the AO, if there is one.

    B.B.

  • Gordon McKinney Says:

    Top tip for target shooting… Place your gun on a stable surface, put your eye in the sweet spot move your head around a little. If you see the reticle shift up/down or left/right relative to the target then you need to tweak the AO/parallax ring. Keep doing this until there’s no movement.

    Once completed and assuming your shooting from the same spot you should find your accuracy has increased significantly.

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