Cant errors



Author Topic: Cant errors  (Read 27498 times - 2 votes) 
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Offline Scotchmo

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Re: Cant errors
« Reply #40 on: January 29, 2016, 01:43:02 AM »
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:



"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.
« Last Edit: November 13, 2018, 07:54:14 PM by Scotchmo »
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Offline cpt_sfc

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Re: Cant errors
« Reply #41 on: January 29, 2016, 11:41:48 AM »
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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Offline bandg

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Re: Cant errors
« Reply #42 on: February 01, 2016, 09:36:13 PM »
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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Offline twigboy

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Re: Cant errors
« Reply #43 on: February 13, 2016, 08:20:13 PM »
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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Offline bandg

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Re: Cant errors
« Reply #44 on: February 15, 2016, 10:45:33 PM »
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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Offline 56S

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Re: Cant errors
« Reply #45 on: July 09, 2016, 10:36:18 AM »
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.
« Last Edit: July 09, 2016, 10:38:16 AM by 56S »
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Offline Scotchmo

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Re: Cant errors
« Reply #46 on: July 09, 2016, 02:02:00 PM »
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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Offline 56S

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Re: Cant errors
« Reply #47 on: July 09, 2016, 02:12:10 PM »
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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Offline 56S

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Re: Cant errors
« Reply #48 on: July 09, 2016, 02:14:02 PM »
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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Offline ken

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Re: Cant errors
« Reply #49 on: September 14, 2016, 02:03:18 PM »
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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Offline Scotchmo

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Re: Cant errors
« Reply #50 on: September 14, 2016, 03:57:56 PM »
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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Offline Iamscotticus

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Re: Cant errors
« Reply #51 on: October 08, 2016, 01:24:40 AM »
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! ???
« Last Edit: October 10, 2016, 12:30:24 AM by Iamscotticus »
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Offline nervoustrigger

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Re: Cant errors
« Reply #52 on: October 09, 2016, 11:14:55 AM »
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]
« Last Edit: October 09, 2016, 10:21:41 PM by nervoustrigger »
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Offline Iamscotticus

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Re: Cant errors
« Reply #53 on: October 09, 2016, 11:56:18 PM »
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.
« Last Edit: October 10, 2016, 12:29:55 AM by Iamscotticus »
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Offline nervoustrigger

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Re: Cant errors
« Reply #54 on: October 10, 2016, 01:44:02 AM »
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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Re: Cant errors
« Reply #55 on: October 10, 2016, 02:16:23 AM »
Excellent Jason
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Offline jim p

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Re: Cant errors
« Reply #56 on: December 12, 2016, 11:45:22 PM »
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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Offline Scotchmo

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Re: Cant errors
« Reply #57 on: December 15, 2016, 12:27:27 AM »
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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Offline Bubba Zanetti

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Re: Cant errors
« Reply #58 on: December 22, 2016, 11:15:06 AM »
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.

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Offline Airguns- VA

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Re: Cant errors
« Reply #59 on: June 02, 2017, 10:40:05 PM »
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
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