I saw this on another site a few years back & got permission from the creator to share his CAD drawing here. Many thanks to AGN user @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 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.
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.
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.
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.
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 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.
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.
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.
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.
