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# Reticle Perpendicularity by Darrell Holland

#43
11-02-2010, 07:16 AM
 Bronze Member Join Date: Aug 2009 Posts: 31
Re: Reticle Perpendicularity by Darrell Holland

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.
#44
12-18-2010, 03:24 PM
 Junior Member Join Date: Dec 2010 Posts: 1
Re: Reticle Perpendicularity by Darrell Holland

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
#45
05-15-2011, 10:44 PM
 Silver Member Join Date: Oct 2009 Posts: 227
Re: Reticle Perpendicularity by Darrell Holland

I didn't realize that this thread existed when I created my own - Need a little help with interpreting scope data

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
#46
05-17-2011, 12:15 AM
 Junior Member Join Date: Mar 2011 Posts: 5
Re: Reticle Perpendicularity by Darrell Holland

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!)
#47
06-01-2011, 03:03 PM
 Silver Member Join Date: May 2009 Location: Prosperity, Pa Posts: 364
Re: Reticle Perpendicularity by Darrell Holland

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!!
#48
06-04-2011, 09:05 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,973
Re: Reticle Perpendicularity by Darrell Holland

Quote:
 Originally Posted by paphil 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.
#49
06-04-2011, 04:58 PM
 Silver Member Join Date: May 2009 Location: Prosperity, Pa Posts: 364
Re: Reticle Perpendicularity by Darrell Holland

[QUOTE=Mikecr;513246]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.

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