Reticle Perpendicularity by Darrell Holland

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SBruce: the ACD needs to be lined up with the scope, NOT the gun. If you follow this method the ACD, scope tracking axis, and bore will all be in a nice line, but getting the ACD set to the tracking axis is much more important, and should happen first and independently.

My method for ACD calibration is to set the gun in a vice so the crosshairs are lined up with a building edge or other vertical object. Then I crank the turret to ensure it tracks vertically, in-line with the crosshairs (all my Leupolds and Super Snipers always have). Now install the ACD with a centered bubble, ensuring you don't bump the scope off-level as you go.

After that exercise, you may want to line the tracking axis up with the bore. I eyeball it and it's good enough for me - this is why:

There are several things going on here:

1) Your scope may not track exactly along the lines described by the crosshairs. Thus, lining the crosshairs up perpendicular to gravity with an ACD and dialing a shot may result in error. On high-quality scopes I have never found one measurably out of alignment, but it's common on very cheap scopes.

2) There is some error induced by having the tracking axis of the scope not perpendicular to the bore. However, it is linear with distance, and very small. A 5 degree non-perpendicularity (very noticable to the eyeball) and 2" distance from scope axis to bore axis (very long) will only result in 0.875" error at 500 yards, assuming you sighted in at 100 yards.

3) The real issue is perpendicularity of the tracking axis to gravity, which is accomplished by using an ACD referenced to the tracking axis of the scope. A 5 degree error here (often hard to see against a tilted horizon) produces a 5" horizontal error at 500 yards. If you're using a drop reticle, then you need to ensure the reticle itself, not the tracking axis, is lined up with gravity.

So, lining up the tracking axis (or drop reticle) with gravity is crucial, as we all know. Lining up the tracking axis with the bore is of debatable value for hunting applications. Certainly it's worth getting right, but make sure you have an ACD installed correctly first, since that's a much bigger error.

 

[stu-e-a]

i use the black and decker work mate or bipod and spirit level to start with and then use a chalk line on the paper i find its better than a spirit level on the paper especially if you target has a little movement in it then i am almost using the exact method as in the article

 

[DZelenka]

I didn't realize that this thread existed when I created my own - http://www.longrangehunting.com/forums/f18/need-little-help-interpreting-scope-data-72638/

I would appreciate if you guys (Mr. Holland would be great) would look at the picture posted and comment whether I need to fine tune or leave well enough alone. THanks in advance.

Dan

 

[Hoofprint]

This is an interesting discussion. I have always just "eyeballed" the rifle/mounts level & sighted on a distant vertial pole, side of a building, etc. But recently I did this as usual, & my son thought I had aligned the reticle at a slight angle. He is left-handed, so I thought he must just be holding the rifle up differently to me. But when I took it out to the range, I could clearly see that it was a few degrees out. So next time I will put a spirit level on the mounts with the rifle in my gun vise & hang up a plumb bob 30 yards or so away. I think that will be an improvement on "eyeballing" which let me down this time, and it also will be cheap! (like me perhaps!)

 

[paphil]

I've somehow gotten sidetracked into this thread while reading another thread and just want to add my 2 cents on this subject. I have used Darrels inverted T in an effort to purposely set up my scopes to shoot 1 inch left of the line when dialed up to 1000 yards to eliminate spin drift from the equation of doping for the wind. I started out thinking I could cant the gun and hold the scope vertical but this failed. As long as the scope was vertical, no amount of rotation would make the gun shoot right or left at the top of the T . The scope itself must be canted to move the POI . Went back to using a level to set the gun straight and put scope on level and then used my scope level to cant scope and gun 3 degrees while the scope level remains level. Now when I aim at the T my reticle is not square with the verticle T line , it leans left by 3 degrees, and my POI is 1 inch left when turned up to 1000 yards. Still dead on at 100 while compensating for spin drift. I must use the level for all shots . This T test will tell you if the adjusting mechanism and reticle are not parallel inside your scope but no amount of rotating the scope will fix the problem. If your scope is vertical and shoots a little left , you have a winner for long range shooting!!

 

[Mikecr]

This T test will tell you if the adjusting mechanism and reticle are not parallel inside your scope but no amount of rotating the scope will fix the problem.

I disagree.
I want my elevation ADJUSTMENT plumb. This is provided with scope turning and then the scope level is set as 'level' with the gun in normal shooting position. I couldn't care less about how the crosshair 'looks' afterwords.

Spin drift compensation(but also coriolis) is built into this setting if adjusted at range.
I don't do this, but dial them in/hold-off as seperate and changing components(which they are).
I want vertical AND horizontal clicks plumb(with no bias).
If I need 1click up & right at 900yds to account for spin drift at a given wind, I dial it.

 

[paphil]

I disagree.
I want my elevation ADJUSTMENT plumb. This is provided with scope turning and then the scope level is set as 'level' with the gun in normal shooting position. I couldn't care less about how the crosshair 'looks' afterwords.

I agree with you on this. The adjustment mechanism must be plumb for the gun to shoot a vertical string with all shots hitting the vertical line. The reticle does not always fall parallel with the adjustment mechanism and as you say, the scope level must be used to set the mechanism to vertical. The reticle can appear canted and the scope will still track verticaly. Most good scopes will be parallel.

My idea of canting the mechanism 3 degrees to the left to eliminate spin drift works only when using a scope level to make sure the cant is consistant. Doping for spin drift is fine but any variable I can eliminate makes shooting that much easier. I'm sure Darrell knows what he is talking about, but he doesn't mention using a level.

 

[Mikecr]

I probably misunderstood you, and I agree that a scope level is critical.

I don't feel that just any(of the many) are right for the application though.
The only version I use/will use, is this: Scoplevel Anti Cant Leveling Device
It's not fancy, some think too cheap, but I haven't had one fail yet.
It allows me to level my scope ADJUSTMENT, independent of the gun.
In fact, once set on a scope, I can move that scope from gun to gun without a need to adjust level -ever again.
I can also see it from shooting position, straight forward of my shooting eye/axis, without affecting cheek weld.

I've been waiting for & surprised that nobody has made a metal 'tactical' version, with switchable LED lighting of the bubble.
Gonna jump on that if they do it right.

 

[Grijim]

What a great article. excellent refresher. Thank you Darrell.

 

[Matthew Courtney]

Even a reticle that is canted 45 degrees is as perpendicular to the bore as any other. The entire sight plane is roughly perpendicular to the bore on a correctly mounted scope, without regard to the reticle's position on the sight plane. The horizontal axis of the reticle needs to be level when the rifle is in the shooting position.

The explanation of the Coriolis effect is incorrect as well as only points on the equator move rotate at that speed. The author leaves out the rest of the globe where rotational speed on the earths surface decrease as points near the poles. The main impact of the Coriolis effect derives from the truth that points at differing distances from the equator are moving at different speeds, thus unless the shooter and target are at precisely the same latitude, one must account for the differing relative speeds of the shooter and the target.

At least the solution described addresses a common issue, incongruent rifle/scope cant, albeit not the issue that the writer described.

 

[Matthew Courtney]

"Gentlemen,


Yes, we see alot of shooters who cant the rifle. The easiest cure is to purchase a scope level and mount it on the scope tube ( do so when we are making the reticle plumb with the rifle in the vise) This is a great truth detector and will aid you in long range shooting. If Len okays it, I'll do an article as to the effect of reticle cant at long range.

Sincerely


Darrell Holland"


"Cure"? Seriously, "Cure"? As if canting a rifle is a disease? I would submit that if a shooters execution of the fundamentals is best done with a canted rifle, that it is a narrow attitude that "what is best for me is best for everyone" is more of an issue than said canted rifle. If the cant of the rifle is congruous with the cant of the scope upon it, the maximum total detrimental effect of such is the distance of the scope height over bore(2 inches or less on most bolt Guns, 2.6 inches on most AR's) AT ANY DISTANCE. This is while every miscue in the correct application of the fundamentals is magnified by the distance, as are errors in estimating wind, range, and most other variables. When the max error for something is 2-2.6 inches at 1500 yards, I would not put that thing before the correct execution of the fundamentals and developing mastery of wind and range estimation.

 

[bigngreen]

"Gentlemen,


Yes, we see alot of shooters who cant the rifle. The easiest cure is to purchase a scope level and mount it on the scope tube ( do so when we are making the reticle plumb with the rifle in the vise) This is a great truth detector and will aid you in long range shooting. If Len okays it, I'll do an article as to the effect of reticle cant at long range.

Sincerely


Darrell Holland"


"Cure"? Seriously, "Cure"? As if canting a rifle is a disease? I would submit that if a shooters execution of the fundamentals is best done with a canted rifle, that it is a narrow attitude that "what is best for me is best for everyone" is more of an issue than said canted rifle. If the cant of the rifle is congruous with the cant of the scope upon it, the maximum total detrimental effect of such is the distance of the scope height over bore(2 inches or less on most bolt Guns, 2.6 inches on most AR's) AT ANY DISTANCE. This is while every miscue in the correct application of the fundamentals is magnified by the distance, as are errors in estimating wind, range, and most other variables. When the max error for something is 2-2.6 inches at 1500 yards, I would not put that thing before the correct execution of the fundamentals and developing mastery of wind and range estimation.

You need to re read that whole article there is a lot in it for your to learn!!!!

 

[Matthew Courtney]

You need to re read that whole article there is a lot in it for your to learn!!!!

There is some good, accurate information in the article, but nothing that I did not already know. The good things in it are eclipsed by partial and incorrect information. Can you actually refute any of my points, or is hurling insults the only thing that you can do?

 

[Greyfox]

I border on the verge of neurotic when it comes to scope alignment, and the use of a scope level for LR shooting. Whether for hunting or the tactical sports there are shooting situations where even the best eye will have trouble achieving a level rifle position without a level. I like the Holland and Flatline scope levels.
While I have tried the system of adjusting the angle necessary to compensate spin drift I found that the crooked appearance when mounting, then correcting with the level, was distracting compared to just the checking glance of the trued bubble level. Out to my 1200 yard hunting max, spin drift corrections are easily corrected from memory or handled by the BC.
I have tried the various devices over the years and found that using the simple/cheap Wheeler level kit to match the action, scope turret, and scope level for the last dozen or so rifles were perfectly aligned when tested on a plumbed tall target at 100 yards. As has been mentioned, the biggest watchouts are to confirm that the top of the scopes elevation turret is perfectly perpendicular/parallel to the crosshairs; and, that the verticle crosshair is aligned with bores centerline. While testing on a plumbed tall target it is also a good opportunity to confirm the accuracy of the scopes click values.