All Springer/NP/PCP Air Gun Discussion General > Optics, Range estimation & related subjects

Cant errors

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Cant errors can be tricky as we try to compensate for them but end up introducing other errors.

I posted this information on another forum after seeing some questions and confusion about cant errors. I thought about putting this in the Optics child board, but scope cant is only one type of cant. Scope cant is not the same as gun cant but the symptoms can be mistaken for each other. I addressed them both in order to show the difference. I had a request from another GTA member to post on this forum. I hope readers here find it useful.

This is some esoteric cant information that I have been thinking about for the past couple of days. There are a number of different cant combinations. How far left or how far right the misses are will depend on the degree of cant, so that could be another subject. The goal is no cant. Hopefully this diagram can help diagnose and solve some of the cant induced errors that I and other readers may experience from time to time. Because of the unique conditions that FT shooters must deal with, we are the ones most likely to be interested in it. Early on in my attempts to shoot airguns with precision, I was convinced that the pellets must be spiraling. Spiraling is certainly a possibility, but from what I know now, it was likely cant errors. In all instances, the gun in the diagrams would be zeroed at the apex of the trajectory.

Thank you Scott ...
Really felt this info quite important as general FYI to all who shoot with optics  8)

I'd imagine it is a pain to diagnose cant errors sometimes. Thanks for the info!

Scott, that is the BEST analysis on Cant errors I have ever seen.... is it something you came up with yourself?.... It's BRILLIANT !!!

My Hatsan AT-44S Long in .25 cal has error # 2 in the bottom row, when you hold the crosshairs horizontal, the vertical crosshair is to the right of the bore centerline.... It shoots exactly as you stated, near shots left, far shots right.... I wrote a thread about this which I published on several Forums, and was promptly told that if the crosshairs were horizontal there could be no such effect.... that I must have been holding the crosshairs not level....

It's nice to see somebody confirm what I said.... Many thanks.... It would appear that setting the vertical crosshair so that it intersects the bore centerline is the first important step.... After that, hold the crosshairs level with the horizontal and you're good to go....



When I finally got to a certain level of shooting precision, I started to notice these cant errors. I thought about it for quite awhile before I decided to just lay it out on my CAD program to see what was happening. I read a lot of information (and misinformation) on the internet about cant errors. The above diagram is a compilation and consolidation of that information (minus the misinformation). 

We cannot assume that every gun is setup correctly. As you saw, it is possible to be holding the gun so that the scope is leveled to the earth, but the vector between the bore and the crosshair centerline may still be canted. Two problems than exist - The scope is canted in relation to the bore AND the gun is canted in relation to the earth.

If we simply line up a bubble level with the reticle, than we are assuming that everything else is in perfect alignment. It never is.

Getting the vertical line of the crosshair in line with the bore is normally the best first step before attempting to use a bubble level. That is usually what I attempt to do these days. Otherwise it can get confusing. However, it might depend on the type of bubble level that you use. I use both types but I like the type that mounts on the scope tube and can be rotated. They are the easiest type to adjust. The type that lines up with the scope mount or directly on the dovetail requires that the scope mounts be centered fairly well on the dovetail, otherwise they can be difficult to adjust. With the prefered scope tube mounted bubble level, you could mount it first and use a plumb line to tie it to the reticle. And then rotate the scope and bubble level as one, while in the rings, a little at a time until all shots are in line. I have done it that way as well. If your close shots were hitting left, and the far shots were hitting right, the above diagram would indicate that you need to rotate the scope and bubble level CW.

An important imaginary line is the vector between the crosshair centerline and the bore. I finally realized that the purpose of the gun or scope mounted bubble level is to keep that vector vertical in relation to the earth (to eliminate gun cant). The orientation of the reticle (scope cant) is secondary, and both have to be correct for the trajectory to line on all aim points, at all distances.


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