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My rifle scope goes off-centre when I change magnification

George Wallace advises a reader who is having scope trouble

Once aligned with the scope’s 20m zero, the GL22’s laser appears above targets that are closer and below those that are further away

Q: Every time I change the magnification on my scope, my rifle shoots 2in away from the cross-hairs. Can you explain what is happening and why my scope goes off-centre?

A: You have a problem that is common to scopes that have a zoom magnification ring to adjust the scope’s power. Usually this is associated with cheaper scopes, though expensive ones can suffer if ill-treated. (Read how to choose a riflescope.)

You have to think of a scope as a tube within a tube. The outer body supports the lenses and the thinner or erector tube, which moves to change elevation/windage, magnification and power, sits in the centre supported by springs and bushes.

Obviously every part of the scope needs to work smoothly and in unison and, of vital importance, it should consistently return to the same starting point throughout its adjustment.

Scope goes off-centre on cheaper models

However, cheaper scopes tend to skimp on quality materials and often parts do not work well together. (Read two of the best scopes for £1000 and under.)

Typically, the elevation/windage adjustment can vary, or the rifle can lose zero from one day to the next if the temperature changes.

Quite often, and this is what you are experiencing, the zoom ring construction, often on the same inner tube, will move the whole erector tube as you change the magnification.

This will then obviously affect the windage or elevation and shift your zero. (Read how to zero your rifle.)

The only answer is to replace the scope with another one to check that this is the problem and, if so, either to replace or repair the faulty scope.

I hope that helps. (Read what you need to know about focal planes.)

This article was originally published in 2014 and has been updated.