Cant errors

[Scotchmo]

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.

[Motorhead]

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

[UCChris]

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

[rsterne]

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....

Bob

[Scotchmo]

Bob,

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.

[rsterne]

If the lower extension of the vertical reticle is through the center of the barrel, and that line is also vertical when you shoot, then nothing else really matters.... In fact, the action (and stock) could be laying on it's side, with the scope mounted on the side of the rifle, but vertically above the boreline and the gun would still shoot perfectly.... It might look weird, but no cant error would occur.... I think I'm going to make a device to insure all future scope installations will have the vertical scope reticle passing though the center of the boreline.... As you say, that's the starting point....

Something like this.... http://www.projectsavage.com/?q=content/exd-engineering-vertical-reticle-instrument

Bob

[Scotchmo]

Bob,

It sound like you now have a good handle on the concepts.

There are tools that are made for this purpose. I just use a mirror set at 5 to 10 yards down range to do the same. Either will get you very close. I can correct for most of the scope cant. But unless the reticle is perfectly centered on the objective bell, there may still be a slight cant. The ultimate verification is while shooting various distances at the range.

Mirror method:
I stand a mirror down range so that I can look through the scope and see the scope objective and the end of the bore in the mirror. I then rotate the scope in the mounts until the crosshair in the scope intersects the center of the bell housing and the bore at the same time. That gets me in the ball park. On low, one piece mounts, the bottom of the turret housing can sometimes interfere with the mount before the scope can be rotated enough. On at least one of my rifles, I just live with the minor cant error that remains. The better way might be to machine some additional clearance into the mount.

When I say "gun" cant, I'm referring to that vector between the center of the scope and the center of the bore. The stock, the grip, the scope mounts - none of that matters in the final determination of cant. The stock and scope mounts can be twisted and warped any which way, and as you said, even sideways.

I use the mirror to correct the "scope cant" and then lock down the scope mounts, then I can use a plumb line (or carpenters level or edge of a building) to adjust the bubble level and tie it to the reticle. Then, when I am in the field, I use the gun mounted bubble level to eliminate "gun cant".

"Scope cant" happens from an incorrectly mounted scope. "Gun cant" happens when I don't hold the "gun" perpendicular to the earth. 

[Scotchmo]

The following is a procedure for eliminating both types of cant errors.

Adjust out any scope cant (important for any shot requiring holdover, close or far):
1) Sight in at the trajectory apex (usually 23-32 yards, 25 yards is good)
2) At 10 yards, shoot at a target that has a vertical line on it. Use a plumb line or construction level to insure it's vertical.
3) Rotate the scope in the mounts until all shots hit exactly on the line at 10 yards.
4) recheck the 25 yard zero and repeat until zeroed at 25 yards and on the line at 10 yards.

After that, your scope should be locked down and securely tightened in the rings. If you plan on using a gun mounted bubble level, you still need to adjust that.

Eliminate gun cant (especially important for far shots):
1) line the reticle up with the vertical line on the 10 yard target (or use a plumb line at any distance).
2) Rotate or adjust the gun or scope mounted bubble level until the bubble indicates level while the reticle is still on the plumb line. Lock the bubble level in that position.
3) Make sure the bubble indicates level whenever you shoot.

There is not much point in eliminating gun cant until you are confident that the errors are not related to scope cant. So, adjust out the scope cant first. Scope cant is eliminated by adjusting the scope in the rings. And then gun cant is eliminated by holding the gun and scope level.

[Powder burner]

    Good stuff.   Thanks for the info, Scott.

[calixt0]

The following is a procedure for eliminating both types of cant errors.

Adjust out any scope cant (important for any shot requiring holdover, close or far):
1) Sight in at the trajectory apex (usually 23-32 yards, 25 yards is good)
2) At 10 yards, shoot at a target that has a vertical line on it. Use a plumb line or construction level to insure it's vertical.
3) Rotate the scope in the mounts until all shots hit exactly on the line at 10 yards.
4) recheck the 25 yard zero and repeat until zeroed at 25 yards and on the line at 10 yards.

After that, your scope should be locked down and securely tightened in the rings. If you plan on using a gun mounted bubble level, you still need to adjust that.

Eliminate gun cant (especially important for far shots):
1) line the reticle up with the vertical line on the 10 yard target (or use a plumb line at any distance).
2) Rotate or adjust the gun or scope mounted bubble level until the bubble indicates level while the reticle is still on the plumb line. Lock the bubble level in that position.
3) Make sure the bubble indicates level whenever you shoot.

There is not much point in eliminating gun cant until you are confident that the errors are not related to scope cant. So, adjust out the scope cant first. Scope cant is eliminated by adjusting the scope in the rings. And then gun cant is eliminated by holding the gun and scope level.

never thought of doing it this way.. genious

[Geoff]

thanks for the info

tagging for when I have the time to check mine

[TimmyMac1]

never thought of doing it this way.. genious

[/quote]

Scott really is a smart guy and he has changed how people shoot in the Hunter Class he and his son dominate. Allen Hull just took the Overall win at the Western Regional FT Match from the Hunter Class. Scott and Allen have raised the Bar in California and he teaches people stuff everywhere he goes. He is a great source of how to win in FT. WFTF guys could learn a thing or two about Ranging from Scott and he is so generous with his research he is helping the competition understand how he can be so good. Scott merges Science with the Art of FT and comes to some very imaginative solutions, that all work.
He has made a huge contribution and he is one of the nicest guys you could meet.

TimmyMac1

[Motorhead]

never thought of doing it this way.. genious

Scott really is a smart guy and he has changed how people shoot in the Hunter Class he and his son dominate. Allen Hull just took the Overall win at the Western Regional FT Match from the Hunter Class. Scott and Allen have raised the Bar in California and he teaches people stuff everywhere he goes. He is a great source of how to win in FT. WFTF guys could learn a thing or two about Ranging from Scott and he is so generous with his research he is helping the competition understand how he can be so good. Scott merges Science with the Art of FT and comes to some very imaginative solutions, that all work.
He has made a huge contribution and he is one of the nicest guys you could meet.

TimmyMac1

Indeed he is ... got some one on one tutoring from him at the Western States match last weekend.
Scott even gave me a copy of his basic Bracketing Chart for the Gamo targets & 8" base blocks used in the match.

Very generous and hiding nothing ... just Wow 

* spell correction

[rsterne]

I finally had the opportunity to set up a mirror and check the alignment of the vertical crosshair to the barrel as outlined by Scott above.... I have about a dozen airguns with scopes, and three of them were out and had to have the scope rotated in the rings.... The one that was the worst was the Hatsan, and it is the one that I had the trouble with shooting to the right at far ranges and to the left at close ranges.... The situation was exactly like the second drawing in the bottom row, and to correct the scope cant I had to rotate it clockwise in the rings (when viewed from the back) to get the crosshair to intersect both the center of the objective and the cented of the barrel when viewed in the mirror.... I expect that will cure the problem I had, providing I hold the crosshairs level, of course.... That is something I don't seem to have a problem with.... I have an eye and brain that can see and abhors crooked pictures on the wall, or tilted reticles....

I really appreciate the drawings and analysis, Scott, and from now on I am going to use a mirror to check my scope to barrel alignment any time I install a scope.... Many thanks for the tips....

Bob

[Sfttailrdr46]

    An old dog just learned a new trick today and to see that the guy who gives me the biggest brain pain other than Pacprotector didn't know something gun related made my evening, Thank you Scott    Just kidding  Bob I love your threads just forces me to think more than I am used to

[Dockey 454]

   Thanks Scott, good stuff. I have a new scope for the HW100, going to try your method.
    Tom

[TimmyMac1]

  An old dog just learned a new trick today and to see that the guy who gives me the biggest brain pain other than Pacprotector didn't know something gun related made my evening, Thank you Scott    Just kidding  Bob I love your threads just forces me to think more than I am used to

We have all learned a thing or two. It is good to have so many that are helpful. Scott is a very brainy guy and he shares with everyone. He is passionate about developing solutions for FT competitors of all classes. He raises the bar which is always appreciated IMO.
The knowledge shared here is priceless because you will never know it all. Nobody can.

TimmyMac1

[Yarp]

Thank you.
Major bookmark.
I feel like I've gone back to school.

[DD789]

one question with Scott's technique for adjusting cant error. is the 10 and 25yrd. distance measured from the muzzle, breech or front bell of the scope? I'm thinking from the muzzle.

[TimmyMac1]

one question with Scott's technique for adjusting cant error. is the 10 and 25yrd. distance measured from the muzzle, breech or front bell of the scope? I'm thinking from the muzzle.

Choose one and stick with it. Doesn't really matter as long as your sight dope is using that as the starting point. When you scope range find the front bell is what is being used to focus with so that is the starting point.

TimmyMac1