GTA
All Springer/NP/PCP Air Gun Discussion General => Air Gun Gate => Optics, Range estimation & related subjects => Topic started by: Scotchmo on July 31, 2014, 12:11:02 AM
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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.
(http://www.scotthull.us/photos/bucket/CANT-00.JPG)
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Thank you Scott ...
Really felt this info quite important as general FYI to all who shoot with optics 8)
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I'd imagine it is a pain to diagnose cant errors sometimes. Thanks for the info!
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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
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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.
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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 (http://www.projectsavage.com/?q=content/exd-engineering-vertical-reticle-instrument)
Bob
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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.
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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.
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Good stuff. Thanks for the info, Scott.
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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
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thanks for the info
tagging for when I have the time to check mine
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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
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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 8)
* spell correction
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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
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;D :o 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 ;) ;) ;D Just kidding Bob I love your threads just forces me to think more than I am used to
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Thanks Scott, good stuff. I have a new scope for the HW100, going to try your method.
Tom
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;D :o 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 ;) ;) ;D 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
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Thank you.
Major bookmark.
I feel like I've gone back to school.
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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.
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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
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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
Thanks that makes sense.
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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.
It does not matter.
It doesn't have to be exactly 25 yards. Just try to be within a couple of yards of the trajectory apex. 30 yards might be better if you use a high scope.
It does not have to be 10 yards either. Just get as close as practical and still be safe. As the closer distances will require more holdover, it will exaggerate any scope cant errors.
The exact distance measurements are critical when calibrating for range estimating. But this is for cant errors, so exact distance measurements are not important.
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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.
It does not matter.
It doesn't have to be exactly 25 yards. Just try to be within a couple of yards of the trajectory apex. 30 yards might be better if you use a high scope.
It does not have to be 10 yards either. Just get as close as practical and still be safe. As the closer distances will require more holdover, it will exaggerate any scope cant errors.
The exact distance measurements are critical when calibrating for range estimating. But this is for cant errors, so exact distance measurements are not important.
Thanks Scott for the additional information. I guess I was over thinking about adjustments.
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This is some really great info, I will be re-adjusting my scopes tomorrow for sure. I have a laser lever that will be perfect for the job! I often get times when my POI is on at close range and off at longer ranges, now I know how to correct the error using the proper adjustments and tools.
As for setting Distances as some have mentioned I do understand it doesn't matter as long as it is 10 +/- or 30 +/- a couple yards, I site in at 40 yards zero on my guns depending on the FPS or the weight pellet I am using. Anyway what I am really trying to say is I never measure my distance to the target from the muzzle, what good would that do (nothing) in my opinion I am not going to step in front of my muzzle every time I check a distance. After all, I do use a laser range finder at my eye not from the end of the muzzle. Besides 2 thirds of a yard isn't going to make a differnce anyway, unless you shooting past 2 or 300 yards at a low velocity!
My Skil Laser Level... I may have to wait until a cloudy day or just before sunset due to it my not show up in bright daylight. Works great indoors. I have woods where I shoot so maybe it will work ok in daylight!
http://www.youtube.com/watch?v=7BhrLyr7uLk&feature=youtu.be# (http://www.youtube.com/watch?v=7BhrLyr7uLk&feature=youtu.be#)
(http://ecx.images-amazon.com/images/I/41Ku9FLWkxL.jpg)
Thanks for the info Scott and everyone else as well :D
William
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This is some really great info, I will be re-adjusting my scopes tomorrow for sure. I have a laser lever that will be perfect for the job! I often get times when my POI is on at close range and off at longer ranges, now I know how to correct the error using the proper adjustments and tools.
As for setting Distances as some have mentioned I do understand it doesn't matter as long as it is 10 +/- or 30 +/- a couple yards, I site in at 40 yards zero on my guns depending on the FPS or the weight pellet I am using. Anyway what I am really trying to say is I never measure my distance to the target from the muzzle, what good would that do (nothing) in my opinion I am not going to step in front of my muzzle every time I check a distance. After all, I do use a laser range finder at my eye not from the end of the muzzle. Besides 2 thirds of a yard isn't going to make a differnce anyway, unless you shooting past 2 or 300 yards at a low velocity!
My Skil Laser Level... I may have to wait until a cloudy day or just before sunset due to it my not show up in bright daylight. Works great indoors. I have woods where I shoot so maybe it will work ok in daylight!
http://www.youtube.com/watch?v=7BhrLyr7uLk&feature=youtu.be# (http://www.youtube.com/watch?v=7BhrLyr7uLk&feature=youtu.be#)
(http://ecx.images-amazon.com/images/I/41Ku9FLWkxL.jpg)
Thanks for the info Scott and everyone else as well :D
William
+- a few yards at 30 yds is next to no adjustment but +- at 10 yards is huge click value and you need to be within a 1/2 yard with focus distance or you could miss the tiny zones in FT. From 22-40 the adjustments are just a few clicks but when you get below 12 yards the click value per yard is major.
TimmyMac1
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This is some really great info, I will be re-adjusting my scopes tomorrow for sure. I have a laser lever that will be perfect for the job! I often get times when my POI is on at close range and off at longer ranges, now I know how to correct the error using the proper adjustments and tools.
As for setting Distances as some have mentioned I do understand it doesn't matter as long as it is 10 +/- or 30 +/- a couple yards, I site in at 40 yards zero on my guns depending on the FPS or the weight pellet I am using. Anyway what I am really trying to say is I never measure my distance to the target from the muzzle, what good would that do (nothing) in my opinion I am not going to step in front of my muzzle every time I check a distance. After all, I do use a laser range finder at my eye not from the end of the muzzle. Besides 2 thirds of a yard isn't going to make a differnce anyway, unless you shooting past 2 or 300 yards at a low velocity!
Thanks for the info Scott and everyone else as well :D
William
+- a few yards at 30 yds is next to no adjustment but +- at 10 yards is huge click value and you need to be within a 1/2 yard with focus distance or you could miss the tiny zones in FT. From 22-40 the adjustments are just a few clicks but when you get below 12 yards the click value per yard is major.
TimmyMac1
Thanks Timmy,
You are correct thats for sure, I think my wording is a little off, I just meant that as long as I do the testing and adjustment for the cant is at those approx distances and use the same distance each time for the settings. Still dont think I am wording it right.
My scope is 1/8" click at 100 yrds so the clicks are even more major as far as the amount of clicks, they are a little more precise!
William
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Great read!!
Though another question....I have had my scope leveled with the rifle, and everything is good. But as soon as I start to snug down the rings the scope moves and throws everything off. I am using some bkl double strap mounts on the rifle. Just trying to figure if there is an easier way to avoid this or not.
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Normally with scope rings, if you tighten alternate sides, a small amount at a time, you can avoid that....
Bob
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I read this stuff, and I am even more confused than ever. :-[
I sometimes think I am in the wrong hobby. This is not for a complete klutz like me. :(
Maybe I should go back to iron sights. My QB-58 has decent irons and I like it that way.
How about red dot sights? Are they as complicated as scopes are to align?
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Good info indeed.... I have a mirror at home that is just sitting there taking space. I now have the perfect use for it.
Thanks a ton.
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as always, you all make me think a bit more. NOW I have to get a steady-rest ----- $$$ --- and I will have something to do this winter. thanks for the info!
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Another helpful hint I just ran across.... If you center the windage adjustment optically (by counting clicks is close enough) BEFORE installing the scope and then do the mirror trick to align the vertical crosshair to the bore, when you do your final sighting in you will be close to the optical center of the scope, at least for windage.... This will maximize the vertical adjustment range of the scope and make sure that the centering spring that holds the reticle in place isn't near the end of it's adjustment....
Bob
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Have read on a few different places to use shims or something else that is a uniform size. Place them under your turret body, most scopes are flat here, and tighten the rings. If your base is flat, as well as the scope, you will have the scope perfectly level. I took the scope off of my powder rifle to give it a try.....and sure enough it worked great. I have a set of feeler gauges that I used for that scope, and a set of playing cards for my airgun.
Cheap and quick, and it seems to work.
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Ok, I'm lost. Rotate scope in the mounts until all shots are on the vertical line??? How are my shots going to be on a vertical line if I am rotating the scope in the mounts??? As soon as I rotate my scope, won't the elevation I dialed into my scope to zero it cause the shots to go off the vertical line? I'm not sure the point here. If I am centered at 25 and then come to 10 and my first shot hits the vertical line, why would I start rotating the scope? Are you saying if, when I come to 10 yards, my shots don't hit the line, then start to rotate?
I suppose if my scope were perfectly optically centered and I was able to zero using adjustable mounts (not likely) then I could rotate my scope and have the shots stay on the vertical line.
What am I missing?
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Ok, I'm lost. Rotate scope in the mounts until all shots are on the vertical line??? How are my shots going to be on a vertical line if I am rotating the scope in the mounts??? As soon as I rotate my scope, won't the elevation I dialed into my scope to zero it cause the shots to go off the vertical line? I'm not sure the point here. If I am centered at 25 and then come to 10 and my first shot hits the vertical line, why would I start rotating the scope? Are you saying if, when I come to 10 yards, my shots don't hit the line, then start to rotate?
I suppose if my scope were perfectly optically centered and I was able to zero using adjustable mounts (not likely) then I could rotate my scope and have the shots stay on the vertical line.
What am I missing?
Rotate scope in the mounts until all shots are on the vertical line?
YES
How are my shots going to be on a vertical line if I am rotating the scope in the mounts?
You are rotating the scope into alignment.
As soon as I rotate my scope, won't the elevation I dialed into my scope to zero it cause the shots to go off the vertical line?
It might, in which case you should rezero your scope before you continue.
If I am centered at 25 and then come to 10 and my first shot hits the vertical line, why would I start rotating the scope?
In that case, you are good to go. Don't rotate the scope.
Are you saying if, when I come to 10 yards, my shots don't hit the line, then start to rotate?
YES
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Crucial reading material! Bookmarked...
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Scott and other contributors;
This should be put into a lesson plan and be required reading. I spent a career in military and around arms all my adult life, thought I knew a few tricks. Best solution short of paying an expert (Scott ?), I have ever heard of. Now is a good time to pull rifles from the safe and spend some time on snowy cold days. Thanks to all.
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I've re read this thread a dozen times. And I'm still not sure I get it... But I've got four days off to figure it out. And when I do... I suspect it will take my shooting to the next level.
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Simple trick I use is to line up the center of butt of the gun stock on a straight edge or piece of paper, then look down the front of the scope and line my vertical line on the scope with the straight edge of the paper , then evenly tighten the scope rings , watching the alignment stays true as i tighten (+) . it gets things close some tweaking with the mentioned methods may still be needed but rarely.
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I agree. Over the years I have had people that I have tried to explain this to but to no avail most of the time. I really have a hard time relating this to people who want a laser to shoot with. I still have a few rifles that look, not straight when shouldered by an other person they say your scope is crooked , I stopped trying to explain it.
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I agree. Over the years I have had people that I have tried to explain this to but to no avail most of the time. I really have a hard time relating this to people who want a laser to shoot with. I still have a few rifles that look, not straight when shouldered by an other person they say your scope is crooked , I stopped trying to explain it.
That's why I eventually laid it out graphically. It also makes it easier for me to diagnose problems.
Here is an example that I used a while back:
(http://www.scotthull.us/photos/bucket/fix-cant-00.jpg)
"A" is how many people try to setup a rifle. It is setup wrong. It has both scope cant AND rifle cant. Many people assume that mounts need to be centered on the receiver in order to shoot straight. They think the mounts are the problem. Some mounts such as BKL do a better job of this. But in reality, it's not even needed.
"B" shows the scope mounted correctly. There is no scope cant. But the rifle is not being held correctly, resulting in gun cant.
"C" show the rifle in "B" being held correctly. The scope is mounted with the reticle in line with the bore. And the scope is held centered above the bore. It has NO cant issues.
When shooting, it's best to ignore any bubble level on the receiver. The one on the scope is all that matters.
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Great if you don't mind can I copy and use that. It puts on paper what I try to on words.
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I read a long article by a firearms bench rest shooter a few years ago that described how critical it is to have the scope mounted to the rifle with no cant. The article pointed out that unless the scope was perfectly mounted with no cant then you would constantly have a mixture of both windage and elevation movement of POI with any change to either the windage or elevation turret and that the projectile would cross the vertical line and continue to deviate away from it at increasing distance. The article stated that, without surfaces that are verifiable as level on both the scope and the rifle (and seemingly few rifles if any have such surfaces that can both accept a level and be verified as square), the only accurate way to verify that scope and rifle are in the same plane is to observe movement of point of impact while cranking elevation up and down. The article suggested placing a level horizontal line on paper (using an actual level) and then place a vertical line across it using a square. This forces the scope to be level when the crosshairs are held on the lines. Then, fire multiple groups while cranking elevation both up and down but always maintaining the crosshairs on the lines. The changing point of impact will follow the vertical line up and down and not deviate off of the vertical if the scope is mounted to the rifle with both level. If the scope is canted on the rifle then the center of the group will move across the vertical line as the elevation is moved up and down.
I haven't used this method on an air rifle yet but it works as described on my long range firearms and it is the method I use to be certain that the scope and rifle are both level. It becomes really noticeable at ranges beyond about 400 yards but the concept should be the same with an air rifle, it would just be less noticeable at the shorter ranges used.
I think a similar test for the shorter air rifle distance might be to shoot a group at your zero distance using the truly vertical and horizontal lines on the paper and then move much closer to target and shoot another group. You would expect impact to be low when much closer but it should be on the vertical line. Any deviation left or right of the line would indicate that the scope is canted on the rifle. The lines make you level the scope but the rifle could be canted to right or left below it which would move the point of impact right if scope is canted right on the rifle and left if scope is canted left on the rifle.
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Reading the above makes sense to me (level target line with right angle drawn) but how would one ensure a dot is not canted?
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Probably would be tough but you don't use any dots. For this method, you always use the center of the crosshairs aligned on the verified vertical and horizontal lines you've made with a level and square, matching scopes vertical hair to the vertical on paper and same with scopes horizontal hair and the level line. Then, when you run the elevation TURRET up or down (physically turning the turret) the point of impact should move similarly up or down but should stay on the vertical line if indeed the scope and rifle are in the same plane. To use this method you must physically move the turret up and down and observe point of impact while aiming at the crossing lines and keeping the scope level. It is a method for verifying that scope and rifle are in the same functional plane, not a method for target shooting per se.
Consider the diagrams provided above and imagine a level scope with a rifle canted to the left below it. This would put the barrel slightly RIGHT of the vertical by an amount directly related to amount of cant (also impacted by scope height above barrel). The point of impact would be to right of but gradually approaching the vertical (moving left) at ranges from the end of the barrel (greatest amount off vertical) to zero distance. At zero distance impact should be right on the crossing lines. Beyond zero distance however the point of impact continues to move left and deviates progressively farther left as range increases.
The curved ballistic arc of the projectile will fall along the vertical at all ranges (ignoring outside factors such as wind) only if the rifle and the scope are both level because we instinctively level the crosshairs as much as possible visually. If the rifle is canted left and the scope is instinctively leveled as described above, the ballistic arc is still exactly the same shape (curved downward by gravity) but that arc is now in a plane completely separated from that of the scope. Not critical for short distances or large targets (or even group size) but point of impact falls farther and farther left as distance increases and gets more noticeable at ranges beyond 2 times your zero distance. From end of barrel to zero distance impact approaches aim point then beyond zero distance it deviates the same initial amount but across the vertical out to 2 times zero distance where it would be the same amount left of vertical that it started out right of vertical. But beyond that distance it continues to move further left of vertical.
Is this critical at air rifle distances? If you are shooting at small targets at longer ranges it seems it could easily cause misses but I assume power/velocity/projectile differences mean that most of us don't shoot airguns at proportional relative long distances as often as we do firearms. I zero my 25-06 using 87 grain bullets at 3750 fps at 250 yards and have access to 800 yard range and beyond on occasion. The cant factor becomes quite noticeable beyond about 600 yards and just becomes more noticeable as distance increases. If you zero your airgun at 25 yards then you would have to shoot beyond 60 yards regularly to notice the major effects. If you do that then all the above would probably be beneficial. I personally like knowing mine are level/in the same plane but is it really critical?
I believe that the really important factor with this type of mismatch is wind drift. Think about a right to left wind in the above cant description taking a pellet that started right and moved left and moving it increasingly that way. Now think about a left to right wind with the same setup. This pellet would also start out right and moving left but would be pushed back to the right by the wind. The right to left wind would appear to the shooter to move point of impact more than the left to right wind. Could the two different conditions have differing effects on rate of velocity loss as well? Seems like it would. Wind drift seems to be the largest error most shooters usually have to contend with (distance is measurable and trajectory is usually well known) and anything that could make it more complicated surely could hurt shooting.
Thinking about this stuff always makes my head hurt so I'm gonna have a beverage now. Think about it some and good luck.
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Now I'm going to add to the confusion on scope/rifle cant. My new Crosman Maximus .22 has a scope level set so the crosshairs are plumb and level when the bubble is centered. The scope is aligned to the rifle as best as my old eyes can do. The attached pictures will most likely come out 90 degrees off as they seem to do here. The test was to shoot at one target placed at 45 yards from a rest keeping the scope level bubble centered. The shot string started at 1200 PSI and went down to 800 PSI. My sweet spot is 1800-1200 PSA and lower than that the velocity drops off in an even fashion. No attempt was made to center the scope to the bull prior to this test. The picture without the steel rule shows the shoots going both lower and slightly to the right. The scope was rotated slightly clockwise in the rings and the test repeated. The second target shows the steel rule and the shots string is nearly vertical.
Where I have trouble understanding all this is that I just assumed one had to be shooting at targets placed at different distances in order to see the effects of scope/rifle cant. All I know now is that my Synco now shoots in a nice straight vertical line at targets placed at varying distances on a calm day. All is needed is to adjust the elevation for the distance.
Edit: How do you post pics taken from a mobile device and keep them from being rotated 90 degrees CCW? Please rotate my photos 90 degrees CW in your minds eye.
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565,
A large variation in pressure in a pre-charged gun can cause more than velocity variations. Vertical stringing is the obvious result of the velocity variation. POI can change horizontally as well with the changes in hammer rebound, expansion/contraction of air tube, varied harmonics and pellet dynamics. Lower velocity also means more pronounced wind drift. A slight quartering wind (barely detectable) could easily cause the left to right variance that you show.
I would not base my scope rotation on those results. Better to do it based on tests with a constant velocity, and close shots.
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What I have done since the rotation was to place targets at 20,30,40 & 50 yards and verified the only obvious change in POI was in the vertical. My scope is a BSA Sweet 22 with a very repeatable elevation adjustment and accurate range finding with the AO. A piece of strip of index card around the circumference of the elevation ring allows me to make my own scale for these ranges. I'm aware of the bell curves in velocity on my PCP since after cleaning and charging the first tank was shot over the chrony. That's how I found the sweet spot and why the test was conducted below that pressure range.
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What I have done since the rotation was to place targets at 20,30,40 & 50 yards and verified the only obvious change in POI was in the vertical. My scope is a BSA Sweet 22 with a very repeatable elevation adjustment and accurate range finding with the AO. A piece of double sided tape and a strip of index card around the circumference of the elevation ring allows me to make my own scale for these ranges. I'm aware of the bell curves in velocity on my PCP since after cleaning and charging the first tank was shot over the chrony. That's how I found the sweet spot and why the test was conducted below that pressure range. Regardless it was an interesting excercise.
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Scotchmo,
May I call you or email you direct to discuss your thoughts on "Can Errors" using Aperture Sights?
Ken Douglass
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Scotchmo,
May I call you or email you direct to discuss your thoughts on "Can Errors" using Aperture Sights?
Ken Douglass
Sure - I sent you my email, but your PM is blocked.
I don't use open/iron/aperture sights much. Cant errors might apply when using ladder type sight iron sight. Not so much with a standard aperture sight.
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Sorry i don't have time to resd all this first, but i am sure, that after trying three different ring sets on my Stoeger X20, the ring bases install canted to the right every time. My measurement of the dove tails in relation to the stock is not off, thereis no off center of the gun in the stock. I have my base clamp screws on the left. I use a Wheeler kit with alignment rods, i lap if necessary. Put about an hour of set up work into every set so i don't think im making dumb mistakes here. Im about to give up on this rifle! Help! ???
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I'm not sure I quite follow your description of the problem but what strikes me is your reference to the "scope rails in relation to the stock". You can ignore that relationship altogether when setting up a scope. All that matters is the relationship between the scope and the barrel (or more specifically, the last few inches at the muzzle).
If the scope ends up offset to one side for whatever reason, any impact will be addressed when you align the vertical bar of the reticle to intersect the muzzle. That can be done by looking at the reflection in a mirror.
Take an extreme example example where we have the scope mounted an inch over to the right. As long as it's aligned as described _and_ you hold the rifle "crooked" so the reticle is plumb/level, your trajectory (holdunder and/or holdover) will reliably follow the reticle.
[edit] This morning I was reading and replying from the text-only version, however now that I'm looking at the full version, I see that Scott covers precisely this scenario in reply #40. Check out his diagram and see if that helps. [/edit]
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What is meant by "scope rails in relation to the stock" is that i don't think the rifle body is not vertical or not centered in the stock or the dovetails are off center. I don't think there is any error in the manufacture or assembly, that I can detect.
The examples Scott laid out, #40 A,B,C, don't apply to my problem. In Scotts diagrams, the scope bases are all in alignment with the receiver. If the base is canted, the receiver is too. My problem is I have installed three perfectly new ring sets that have installed canted on a non-canted receiver. And I can't see any obvious defect in the dovetails.
Anyways, I removed the last ring set I installed and rotated the ring pedestals. They are slightly leaning to the left, but not drastically.
Took it out to zero today and found it already very close to center.
I chose not to use the recoil stop screw so I could move the scope as far back as possible.
I initially achieved a 1" group at 41.6 yards with 15.89 JSB domes. Then I started to spread out. Lubricant burning off? I should do better with the GRT III when I get it. Or maybe my bag of lentils is breaking in? I dunno. Do I need to try a bag of rice?
I want to try some lighter domes like 14.3 Benjanin/Crosmans.
I was surprised at how slow the JSBs were. Slow enough for a nutter to react to the springer.
I couldn't get my crony working so I don't have that data.
Id like to add that the moderator on the Stoeger X20s really works. I shot in an area where I had a wall 3 ft on the left and a tall fence 6 ft to my right, for the whole length of the range and I have shot indoors. This gun is quiet. All you hear is the spring, which is jarring but reasonable for a $160 China rifle.
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What is meant by "scope rails in relation to the stock" is that i don't think the rifle body is not vertical or not centered in the stock or the dovetails are off center.
Okay, understood. But in the interest of troubleshooting, I advocate we simplify things by not making the stock part of the equation. It has nothing to do with the success or failure of aligning the things that matter, which are the scope and barrel.
It is good that stock appears to be square to the action because it makes it more natural to hold the rifle in a manner that squares up the scope and barrel to the force of gravity. However it is completely superfluous to setting up the scope successfully.
My problem is I have installed three perfectly new ring sets that have installed canted on a non-canted receiver. And I can't see any obvious defect in the dovetails.
Again, it does not really matter if the scope rings are canted. When you align the vertical bar of the reticle to intersect with the barrel, you have aligned the scope properly. But it does mean that you will then need to purposely hold the rifle canted slightly when shooting as shown in Scott's diagram C. For this reason, a scope-mounted level will be beneficial. The level should be set with the reticle square and plumb to the force of gravity, with no regard to the relationship to the stock.
Regarding the dovetails in particular, are they 11mm type or 3/8"? If 11mm, it should measure very close to 11mm across the wide part of the dovetail, normally within 0.1mm or so. A 3/8" dovetail is dimensioned based on the narrowest part of the groove so it's not as easy to measure. A 3/8" groove will usually measure between 12mm and 13mm across at the top so it is usually easy to distinguish by process of elimination. The other difference is that an 11mm dovetail is cut at a 60° angle and a 3/8" is cut at 45° angle.
If you have tipoff rings with reversible clamping plates, they will likely have one tooth with a noticeably shallower angle than the other. That would be the 60° side you want to use for an 11mm rail. Matching up the correct side of the plate should eliminate this often overlooked source of scope cant with respect to the receiver. Usually it's so small that most would not notice and doesn't matter when the scope is set up properly, but is rather easy to avoid when you know to look for it.
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Excellent Jason
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I am probably over simplifying things but this is how I set up a scope when concerned about cant.
I take a piece of number 12 awg copper wire with white insulation. I bend this wire into a 90 degree angle. I insert one end into the barrel and as I mount the scope I look through the scope and try to make sure that the vertical cross hair lines up over the vertical piece of wire.
There are probably better ways of doing this but this will get you pretty close.
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I am probably over simplifying things but this is how I set up a scope when concerned about cant.
I take a piece of number 12 awg copper wire with white insulation. I bend this wire into a 90 degree angle. I insert one end into the barrel and as I mount the scope I look through the scope and try to make sure that the vertical cross hair lines up over the vertical piece of wire.
There are probably better ways of doing this but this will get you pretty close.
That is a good idea. As long as you can actually see the wire clearly enough.
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Jason:
Thank you! I was having a cant issue related to my scope rings and was beginning to pull my hair out. It was a 11mm/ 3/8 mount issue.
At first I didn't know what it was from and after re-reading this thread pulled everything down and discovered the issue. Or at least I think so ???
When I finally get to shoot the HFT500 I can report back.
BZ
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Being very new to the airgun world I had no idea that there was so much to learn about scopes, mounts, cant. Definitely need to take note of the info if I want to be competitive in FT. Thanks for the great information
Reid
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Hey guys....
Hope all of you are well.
Jes a request to Scotchmo and others.....if you could kindly upload the pics again using some other hosting site as the pics are no longer visible. I actually wanted to guide a friend abt scope cant and was thinking of forwarding him the link to this topic.
Regards.
SORRY GUYS, THERE WAS SOMETHING WRONG WITH MY SERVICE PROVIDER. IT'S ALL GOOD NOW.
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Been shooting AG for about 20 yrs and finally found this site and this thread.
All questions answered at once. Thought I was trying to learn how to move windage adjustable mounts into barrel alignment with a leveled gun reciever. Nope. That's bass ackwards...
But I do want to add to this by suggesting a great lil trick to assure scope mounts are worthy of the effort.
Saw it here on another thread, I take no credit: Mount rings in opposite directions. screws on opposite sides of the rail with the two rings touching to check them for centering on a rail. Cheap mounts may not match up and require you to adjust farther to the edge of the optic center than good ones may.
This is by far the best scope mounting post I have ever seen on any BB...ever. Thanks to you all, even from way back when this started. And yeah if anyone has those photos, this post is definitely worthy of the maintenance.
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Saw it here on another thread, I take no credit: Mount rings in opposite directions. screws on opposite sides of the rail with the two rings touching to check them for centering on a rail. Cheap mounts may not match up and require you to adjust farther to the edge of the optic center than good ones may.
Actually, it is of no particular importance that the mounts be perfectly centered over the dovetail. Your scope can be offset to one side, then you simply rotate it to align its reticle with the bore before you tighten it down. From that point forward, when you shoot, so long as you hold the reticle level, the pellet will rise and fall in line with the reticle.
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I can pretty much guarantee that MOST mounts will not line up, if you fit them with screws on alternate sides.... if for no other reason than there is little standardization on dovetail widths on airguns.... As Jason said, a slight offset is not an issue, as long as the vertical crosshair bisects the boreline....
Bob
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I can pretty much guarantee that MOST mounts will not line up, if you fit them with screws on alternate sides.... if for no other reason than there is little standardization on dovetail widths on airguns.... As Jason said, a slight offset is not an issue, as long as the vertical crosshair bisects the boreline....
Bob
Now I feel really lucky. I had a few sets of older rings and found a nice pair/match. But I get what you two are saying. That being that, for the purpose of reticle VS barrel alignment it won't matter a bit. Plus who knows if the rail is even parallel to the muzzle exit line anyways...just "un-cant" the gun to plumb when you shoot (simply genius BTW, my favorite kind) .... So I do get it.
But all things being equal...and assuming good tolerances on a gun's manufacture, why not try to keep the reticle optically centered as well? Avoiding offset in the rings that MIGHT be avoided, if you can do it, could keep you optically centered better. That is rather than trying to correct with excessive scope windage adjustment that MIGHT be avoidable.
I may even revisit some existing set ups to see if deliberately reversing one mount to the other side can improve my optic centering. They are all getting a vertical reset this way now anyways...
But it may be wise to point out what you did so people don't assume it will always work.
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delete of a repete
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I agree that keeping the scope near its center is useful. There is no use centering it if you mount it on the rifle and then have to crank the turrets nearly to the limits to get it zeroed.
But please understand a scope that’s offset to the left or right has absolutely no negative effect on this goal whatsoever. To clear this up, it sounds like we need to define what is meant by “offset”. When I say offset, I mean the sort of thing introduced by using an 11mm mount on a 3/8” rail. This error is completely erased by simply rotating the scope so its vertical crosshair intersects the bore.
It’s not the same thing as flipping the front ring and rear ring opposite of each other (re: the scenario where the screws are on opposite sides). That is indeed likely to point the scope in a divergent direction from the bore. In general, that’s a big no-no unless you have some other source of error (e.g. bent barrel) that it just happens to help counteract. Also, I should mention you can buy mounts that are self-centering but frankly they are a solution to a non-problem when you know how to align the scope to the bore as previously described.
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Additionally, if the vertical crosshair intersects the bore with the scope optically centered.... and you hold the crosshairs level when shooting.... the only time you will have to dial in windage is if the dovetails are not parallel to the bore.... or if the rings are off center (like might happen if you reverse one ring).... The most common error made in mounting scopes is trying to correct offcenter mounting by cranking in windage.... instead of rotating the scope in the rings (while the scope is still optically centered) until the crosshair intersects the bore.... and then canting the rifle if necessary to get the crosshairs level when shooting....
I had several scopes where the windage was cranked over a long ways.... every one of them returned almost to center by first optically centering the scope, and then rotating it in the rings until the vertical crosshair intersected the bore.... Only slight windage adjustment was then necessary to fine tune the zero....
Bob
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Agree and agree. 8)
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BACK UP TOP WITH THIS AGAIN.
It's come up elsewhere once more.
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Great post.
I feel that my Bubble Level forces me to keep the cant of my gun in the same position for each shot.
My groups are much tighter at 50 plus yards with it.
Charlie
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BTT.
This deseves a sticky.
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Glad this popped up. Great post!
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Additionally, if the vertical crosshair intersects the bore with the scope optically centered.... and you hold the crosshairs level when shooting.... the only time you will have to dial in windage is if the dovetails are not parallel to the bore.... or if the rings are off center (like might happen if you reverse one ring).... The most common error made in mounting scopes is trying to correct offcenter mounting by cranking in windage.... instead of rotating the scope in the rings (while the scope is still optically centered) until the crosshair intersects the bore.... and then canting the rifle if necessary to get the crosshairs level when shooting....
I had several scopes where the windage was cranked over a long ways.... every one of them returned almost to center by first optically centering the scope, and then rotating it in the rings until the vertical crosshair intersected the bore.... Only slight windage adjustment was then necessary to fine tune the zero....
Bob
This thread is great and being new to this, I've learned a lot.
Regarding this post from Bob, is the technique of aligning the vertical crosshair of the scope with the bore only effective with a scope that is optically centered or does this still work if the windage turret has already been adjusted? I know I currently have a can't issue (shooting to one side closer than my zero and the other farther than my zero) but do I NEED to reset my scope's optical center before aligning? I understand there are benefits to this but is it absolutely required to have it optically centered for this to work?
Thanks!
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I would optically center the scope before aligning the crosshairs with the bore.... but you don't need to be anal about it.... Counting the clicks and setting the scope in the middle of its travel is good enough.... Then once you use the mirror and align the crosshair with the bore, you won't have to crank in a bunch of windage, and will remain close enough to the optical center of the scope....
Bob
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Did a quick click count optical center. Used the mirror trick to get things aligned, zero’d at 30y (windage was only off by a few clicks) and am now shooting on the vertical line from 10y to 50y.
Brilliant and super easy! Thanks!
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I would optically center the scope before aligning the crosshairs with the bore.... but you don't need to be anal about it.... Counting the clicks and setting the scope in the middle of its travel is good enough.... Then once you use the mirror and align the crosshair with the bore, you won't have to crank in a bunch of windage, and will remain close enough to the optical center of the scope....
Bob
How does the angle of the mirror effect the outcome. I use the mirror method myself; but I wonder how true my alignment is, if I'm not squared off with the mirror correctly.
Also... Have you found that it makes a difference if you align the crosshairs with the bore, versus aligning the crosshairs with the objective lens?
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That is why I miss. It's not because I can't. It cause my scope can't.
8)
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This guy set up two identical scopes with quick release on a 6.5 Creedmoor. One had the scope reticle aligned to the flat on the receiver. The other had the vertical cross-hair aligned to a plumb line, despite the rifle being slightly canted to match the shooters comfortable hold: youtube.com/watch?v=eCoHG23TQcY
The results may surprise you...
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This guy set up two identical scopes with quick release on a 6.5 Creedmoor. One had the scope reticle aligned to the flat on the receiver. The other had the vertical cross-hair aligned to a plumb line, despite the rifle being slightly canted to match the shooters comfortable hold: youtube.com/watch?v=eCoHG23TQcY
The results may surprise you...
At 1000yrds - not surprising. As long as the reticle was held vertical (no gun-cant), the small amount of scope-cant needed to get the rifle comfortable will be insignificant.
With airguns, we often shoot as close as 10 yards. At 10 yards, scope cant issues will be 100x as much as it is at 1000 yards. It's a function of scope_height/distance_to_target. You would not likely see the error at 1000yards, but you would at 10 yards.
Also - if you are shooting at only one distance (whether it be 10yds or 1000yds), it does not matter at all, as long as you are sighted in (elevation and windage) for that one distance.
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Late to the game! but good info! I didn't read every post, so this is just a problem that I've found associated
to scope can't & that is Scope mounts! they can have a adverse effect by tilting the scope to one side. FWIW, the best rings I have used to solve the problem are BKL, I'm sure there are many others, but that's what I use. Also, most 1 piece 11mm mounts are pretty much anti tilt & grip like crazy. I'm talking 11mm! But still, I have seen it happen with pics to. I to use a scope level beyond 35 yrds. Most airguns just won't let you mount the scope low enough to the bore.
Setting your rifle up to eliminate the problem sure makes ya not want to touch it! its a good idea to test your scope out well & make sure your happy with it before going thru the paces.
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Hi all, the pictures are not showing up for me. Am I doing something wrong for me not to be able to see them? Any help would be greatly appreciated. Thank you
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Hi all, the pictures are not showing up for me. Am I doing something wrong for me not to be able to see them? Any help would be greatly appreciated. Thank you
Page one ... no issue can see the chart/s
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That's a no show on my end. I get a empty box with an icon in top left corner. Nothing happens when I hit on it either. All I have to access the internet is a phone. Don't know if that makes a difference or not. I am not that good at using it either!! Thank you for the reply.
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Quoting Scott's original post, with the image replaced by a copy uploaded to the forum...
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.
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Thank you for taking the time to do that. And thank you all for sharing. This is the BEST airgun forum I visit. Again, thank you all for selflessly sharing your hard gained knowledge. That's what makes this forum what it is.
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Not trying to stir up anything, but this issue has a looooonnnnngggg history out there
https://youtu.be/KDtKkS3G_Mw (https://youtu.be/KDtKkS3G_Mw)