Article on Scope can't showing its not a huge deal

[Taso1000]

Jason,

I totally agree.    

I was just trying to inform those that were leveling against an assumed true squared receiver.  All that matters is that we get the scope and bore centerlines in the same vertical plane and be able to repeat that orientation with a properly adjusted scope mounted level.   

Taso

[RAJOD]

Regarding levels, don't spend your hard earned money on a rail-mounted level.  They shouldn't even be sold unless they have some sort of fine adjustment screw for the bubble tube.  Why?  Because this style attempts to level the receiver.  The receiver does not matter.   Depending on how everything fits together, the receiver may very well need to be held slightly off level.

What you want to do is orient the vertical bar of the reticle so it simultaneously bisects both the objective bell and the muzzle (looking at your reflection in a mirror as previously described).  Then fasten your level so it shows level when the horizontal bar of the reticle is level (or when the vertical bar of the reticle is plumb...whichever one you can do more confidently).  To secure the level correctly, you have to be able to adjust it, and one that mounts to the scope tube is easily adjusted.  Those that mount to the rail usually cannot be.

Yes I like the mirror way as then you don't have think about the level of the gun.  But I found it tricky to eyeball in the mirror that my line split the barrel and the large objective exactly.    Seems it could be off -+ 1 degree and it would still look 50/50.   

I think the idea is to eyeball it in the mirror to be close, but the really dial it in you need to shoot at two distances and see if it hits a plumb line target on both distances. 

Have not had time to really test it, only done the mirror part. 

Also the real reason people have to worry about these can't errors is the scope mount height.  if it were inside the barrel (Alien tech there would be no need

the higher the scope is mounted the worse the errors become.   

[nervoustrigger]

Actually if the line of sight were in the barrel, cant error would still be a present because of the pellet's arced trajectory.  If the scope isn't held level, the pellet will diverge from the aim points...because it's following the force of gravity.

Unfortunately there are many similar bits of misinformation about scopes that get cited by well-meaning individuals which makes it rather difficult to sort out what is really true.  I'm sure I was guilty of it somewhere along the way which is why I usually stop and try to explain when I see them repeated.

[bandg]

With respect, you are dead wrong here Jason.  If line of sight were in the bore there could be NO cant error in any manner.  Ballistic arc is always present and always will be for any projectile fired under the influence of gravity and is really totally independent of cant, scope height, or anything else.  Ballistic arc becomes involved with cant as the LOS and the bore are separated.  If as you state line of sight was in the bore then you could spin the bore 360 and never change anything at all.  The cant error is entirely related to some degree of offset (which is there even to a small degree with very low iron sights but becomes worse the more the bore and LOS are separated) between the bore and the line of sight.

[nervoustrigger]

We're both right, we are just talking about two different things.  Re-reading my post, I can see that I was not clear enough.  Sorry.

The scenario I was trying to describe is holding the reticle canted.  In this hypothetical example where the bore and sight are magically on the same axis, they cannot have any misalignment-induced cant...as you correctly state.  However, if I hold the reticle canted, the pellet will diverge from the vertical holdover/holdunder points shown on the reticle.  Imagine an extreme example where I have canted the reticle 45 degrees clockwise.  If I launch a pellet, clearly it will not arc downward and to the left to follow the reticle.  It will follow gravity straight down.

[bandg]

I hope we are on the same wavelength but I'm not certain and I think this may be where many people get very confused about this issue.  In your last post above about canting the reticle 45 degrees you note that the pellet will not magically follow the holdover dots and will simply "follow the gravity".  Certainly accurate but no one would tilt a reticle 45 degrees unless they have bigger concerns than shooting at a target.  But they may tilt it a few degrees without realizing it and this is the problem with cant in that you would probably notice the big amount but maybe not the small amount.  And although the pellet will always follow the same ballistic arc (following the gravity) that arc can change relative position (not it's shape) dramatically in relation to line of sight because you have the additional factor of convergence of bore and LOS.

I've dealt with this shooting long range firearms but air guns don't experience it to the same degree due to limited range.  Imagine tilting the reticle 5 degrees CW but not realizing it.  The bore is now "out to the left" in relation to the vertical LOS plane.  But even though the ballistic arc is physically only curving downward as shaped by gravity, IN RELATION TO LOS it is also moving to the right as well (LOS and bore must converge to hit on a zero point).  The pellet will move toward the LOS vertical plane and eventually cross it moving away further right.  This is not an extreme amount on an air rlfle because we normally shoot at ranges less than 100 yards and most of us well less than that.  But for firearms it becomes a major issue as you try to reach further out.  The projectile will not only be dropping on its constant physical arc but will be moving further and further right of the vertical LOS plane in the above example. 

Now imagine the above issue combined with a changing windage (right to left and then left to right).  One shot the wind would be resisting the pellet movement toward the vertical plane and maybe it wouldn't cross it or not by much.  The next shot the wind would be aiding the movement and it would cross the vertical quicker and move much further right.  This combination is much worse for air guns because of the lower velocity and poor BC of the pellets.  This is why I noted above setting up my long range rifles the way that I do and why I even take the time to do it with my air guns.  Years ago I spent many hours on the range chasing the above combination of errors until I learned to match the ballistic arc with the plane of scope function (elevation up and down) as exactly as possible first and then set my leveling aid up to that verified level point. 

All of this worsens with higher mounting points for the scope (more separation of bore and LOS and thus more convergence angle needed).  And even though our air rifles don't function at the same ranges as firearms, generally we expect to hit very small targets so it can be an issue and I believe very much so on more windy days. 

[nervoustrigger]

Very well articulated, sir. 

I think we are on the same wavelength, and I will concede you have broader real-world experience in the way of long range powderburner marksmanship.  So can you lend us your perspective on a question?  Given the loopy trajectory of a diabolo pellet, doesn't the practical effect of cant manifest at much shorter distances than a high velocity centerfire round? 

I think of cant error as being an error of "my reticle predicts the pellet will fall to this point out there at a distance, but gravity means it will actually fall to a slightly different point".  So the further out at a distance it goes--and the more the pellet drops--those two points diverge more and more.  So I reason a projectile that drops more is in a sense more sensitive to cant error.  Is that valid to your understanding?

[Frank in Fairfield]

Why would you tilt an arm 45 degress in one direction when you could easily tilt it 15 degrees the other way?

Time to return to your video games, I am thinking...

[nervoustrigger]

I'm half gangsta, nuff said.

[bandg]

Not as I understand it.  The cant error must be separated from gravity even though both effect the shot.  The ballistic arc is relatively constant and always curves with the direction of gravity but the cant error can change in both direction and subtle degree.  It is a related but independent parameter.  Consider again the 5 degree CW cant and assume it moves the bore 1/4" left from vertical.  Zero is probably 25 yards for many of us so bore and LOS converge at that point and the pellet will be on the vertical at that range (it would be a little lower than you would expect because the cant lessens the angle of rise a bit as well so that is another complication but probably negligible).  25 yards beyond zero range (assuming no wind effect) the pellet would only be 1/4" off from vertical to the other (right) side but again lower than you would expect.  But how many of us shoot beyond 50 yards regularly?  That is why I think it is less of an issue for an air gun as it is an error that continually increases with distance.  At 100 yards the pellet would be 3/4 inches right of vertical LOS (it moves roughly 1/4" for each 25 yards unless I'm off on this since I haven't done the math) but again even lower than you would expect.  Take this out to 600 yards and realize you could make the error to either side and you have basically a foot of "slop" in the shot under ideal conditions.  Add in any windage effects and you have not only a larger range of error but also the unpredictable adding and subtracting of the windage effect I mentioned in the above post.  I certainly believe that the windage effects would be much greater for the pellet than for a firearm bullet (and generally worse for 177 than 22, etc.) due to velocity and BC and would almost certainly have a large impact on air gun pellets within their normal range but I don't believe it would ever overcome the range component.  I enjoy discussing such issues but it's been a long day and I could have some of this wrong but I think I have these concepts correctly laid out. 

[rsterne]

Here are all the possible combinations of scope cant and rifle cant.... Scope cant is measured relative to the rifle.... Rifle cant is measured relative to a plumb line (ie gravity)....



In the diagram below, "A" is the most common error made in mounting a scope.... "A" only works if the scope is centered over the bore (almost never happens!) when both are level....



If the scope is not centered over the bore, then you must twist the scope as in "B" so that the vertical crosshair aligns with the bore (this is where the mirror helps).... You then hold the rifle as in "C" to eliminate all cant errors.... The pellet rises and falls vertically, under the effect of gravity, starting from the muzzle.... Only if the crosshair lines up with the barrel, and is also held vertical, does the pellet path track perfectly along the vertical crosshair....

until you add wind.... 

Bob

[RAJOD]

Actually if the line of sight were in the barrel, cant error would still be a present because of the pellet's arced trajectory.  If the scope isn't held level, the pellet will diverge from the aim points...because it's following the force of gravity.

Unfortunately there are many similar bits of misinformation about scopes that get cited by well-meaning individuals which makes it rather difficult to sort out what is really true.  I'm sure I was guilty of it somewhere along the way which is why I usually stop and try to explain when I see them repeated.

  Can't error can only happen if you have a scope mounted on the barrel not inside it.  Its round so even upside down the line of sight would be centered, gravity would play the same role in all positions.     But as you know that example I gave was in fantasy land.   

[RAJOD]

Here are all the possible combinations of scope cant and rifle cant.... Scope cant is measured relative to the rifle.... Rifle cant is measured relative to a plumb line (ie gravity)....



In the diagram below, "A" is the most common error made in mounting a scope.... "A" only works if the scope is centered over the bore (almost never happens!) when both are level....



If the scope is not centered over the bore, then you must twist the scope as in "B" so that the vertical crosshair aligns with the bore (this is where the mirror helps).... You then hold the rifle as in "C" to eliminate all cant errors.... The pellet rises and falls vertically, under the effect of gravity, starting from the muzzle.... Only if the crosshair lines up with the barrel, and is also held vertical, does the pellet path track perfectly along the vertical crosshair....

until you add wind.... 

Bob

Yes thanks Bob.  I was looking at that, "Taso linked it"  I like it.   

Is the most common error due to the dove tails not being centered exactly?  Or is it due to scope rings pushing the scope to one side a little.     

I read you had a few guns that failed on the mirror test.   I think you said that one shot left near and right far or something like that.   When you centered the barrel/objective via mirror did it correct those issues?

Also did what distance did you find it easiest to line up the barrel with objective.   I tried in my house at 10 meters from the mirror and I found it a little tricky to know for sure if I bisected that objective 50/50.   Probably due to less than perfect lighting.   

Thanks!

[dan_house]

Regarding levels, don't spend your hard earned money on a rail-mounted level. 

 Because this style attempts to level the receiver.  The receiver does not matter.   Depending on how everything fits together, the receiver may very well need to be held slightly off level.

Agree with you there. But only to that point. I have used rail mounted and scope mounted (have one of each on two different guns).....
I used 5 or six levels when I set the gun up. on the rest, on the receiver on the scope and any where else I can find something level (and if a magnet sticks to it, bonus...), have a Wheeler leveling kit for the barrel.....   I work  them all until they are all in alignment bubble-wise or a majority of them show the same thing.
back to the rail mount, when the scopes shows itself to be level, thats when I put on or adjust the rail mount. and adjust it against known square/level objects. That becomes field level indicator.   Rail mounted levels are fine IF you understand where they can be wrong, and what they are "calibrated" against .

I agree though, just clamping one on wont get you much

[rsterne]

Yes, using a mirror to align the vertical crosshair with the barrel fixed the problem.... My Hatsan was the worst, zeroed at 50 yards, it would shoot 2" right at 100 yards, and 1" left at 10 yards before I used the mirror method.... I set the scope for the closest possible focus, increase the magnification to maximum, and then set the mirror at half the closest focus, so that the image of the muzzle and scope objective is clear when I look through the scope.... If the scope is a 4-12 X and focuses down to 10 yards, I use 12X and set the mirror at 5 yards.... Set the rifle in a solid rest, reasonably level (not critical), and rotate the scope in the rings until when you look through it, the crosshair bisects the image of both the objective and muzzle.... then carefully tighten the rings, checking to make sure it didn't change.... Note you should have the scope reasonably close to the optical center BEFORE you do this.... If you have to move the crosshairs over a lot in windage to sight in your rifle, you will be moving the crosshair away from the image of the muzzle if you check it again later.... I have never found I have to fine tune my scope rotation after this, but likely that is because I'm not a good enough shot to tell that the impact is off 2mm at 25 yards and 5mm at 100 yards.... In other words, a carefully done mirror setup during scope installation is all I need.... After that, if I hold the vertical crosshair vertical when shooting, my cant problems are eliminated....

Bob

[RAJOD]

Note you should have the scope reasonably close to the optical center BEFORE you do this.... If you have to move the crosshairs over a lot in windage to sight in your rifle,
Bob

Nice, that was my next question, should I reset my scope to optical center.   Thanks!   I've learned quite a bit on this forum.   I really like the mirror trick vs leveling the breach.   And it appears to hold up in real world testing.   

I am a little amazed that all the searching I did on scopes they mostly eyeballed it or used a level.  I was never quite happy with that because of too many assumptions.    Many videos the guys just says "I prefer to eyeball it"  and no disrespect to them eyeballing it gets the job done to "Good enough" for most.    The mirror method goes one step further.   Even after I do it all correctly my kid will still outshoot me with an inferior scope/gun and some can't error.

[rsterne]

There is one simple way to completely, 100% eliminate cant error.... Sight your scope in at the distance you are shooting at.... and never shoot closer or further away.... 

That will even work for side-mounted scopes like on a 30-30 Winchester that ejects out the top.... 

Bob

[RAJOD]

There is one simple way to completely, 100% eliminate cant error.... Sight your scope in at the distance you are shooting at.... and never shoot closer or further away.... 

That will even work for side-mounted scopes like on a 30-30 Winchester that ejects out the top.... 

Bob

That is pretty much what I have been doing in the back yard.   I'll do a 50 yard shoot then adjust for that distance and do some great groups.   I might not shoot again for a week and it could be at 35 yards and then I'll just shoot a few sighting shots and I'm good again.  Never had to really think about can't errors.    Just did not have the time to fiddle with it to the level you have done.   

I'd sight for 20-25 yards and just worry about hold over.   10 yards 2 mildots or so 25-35 dead on.   I had to have been pretty close because I did not stray too much side to side.   I think people that shoot bench competition are far more aware of these errors.  I'm slowing progressing to that level.