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Reticle Perpendicularity by Darrell Holland

 
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  #22  
Old 08-21-2008, 12:30 PM
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Re: Reticle Perpendicularity by Darrell Holland

I think Darell is on vacation with his family for 2 weeks.
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  #23  
Old 08-29-2008, 07:18 PM
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Re: Reticle Perpendicularity by Darrell Holland

Quote:
Originally Posted by guns_and_labs View Post
Good article, thank you. The method described will allow me to test a theory that I need a naturally canted reticle. It seems, when I use one of the reticle devices, that the reticle is not plumb when I actually set the rifle up to shoot -- that my natural position is slightly canted -- all measured by whether the leveled target appears to align with the reticle. Now I can test that easily with Mr. Holland's test.

One question: how often is a person's natural position such that a canted reticle is necessary?
I had basically this same question. One further question: Am I reading this wrong or is the suggestion to mount the scope perpendicular to the bore of the rifle? I have never understood how you can position anything in a relationship perpendicular to a round object. Since it is round, it really has no X or Y axis does it? Level is level, and plumb is plumb, right? If a cylinder is rotatated, up and down don't rotate with it, they are fixed in space. The bullet still exits the barrel, and gravity still pulls straight down on it. Any wind still deflects it to the same degree.

I do not mean to appear argumentative, I just don't understand, and wish to. I want to be better at this long range game. So, if the barrel is round, and I naturally cant the rifle, why shouldn't I mount my scope so that it appears perpendicular the horizon to my eye using my natural hold? The canting only rotates the barrel along its longitudinal axis, the bullet still travels the same path, right?

If I need extra elevation, and I dial it in or hold over, the impact will still travel in line with the vertical wire of my reticle because I am moving the canted rifle up to adjust to the new point of aim. The center of the barrel still moves in the same direction - up. Rotating the barrel by canting doesn't change the direction by which gravity pulls the bullet straight down, it doesn't alter the ballistic arc. So, I am still confused as to why a naturally canted hold on the rifle would produce an off shot unless the rifle itself were held perfectly perpendicular to the horizon for that shot. If I cant the rifle but have mounted my scope so that even with that cant its vertical wire is parallel to a plumbline, shouldn't that be OK? I can see where dialing elevation into a scope mounted at an angle to a rifle held perfectly plumb would produce a horizontal dispersion.

But holding a rifle at a slight cant and mounting the scope so that its reticle is parallel to a plumb line even given that cant should have the same effect as an offset type of scope mount such as was used on some M1s during WW2 by military snipers. In effect you are holding the rifle with the scope slightly to one side. Am I wrong? I barely passed high school geometry... Again, no flames, just curiosity and respect for the expertise on this forum.

Last edited by mkollman74; 08-29-2008 at 07:24 PM. Reason: typo
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  #24  
Old 08-30-2008, 07:47 AM
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Re: Reticle Perpendicularity by Darrell Holland

Quote:
Originally Posted by mkollman74 View Post
I had basically this same question. One further question: Am I reading this wrong or is the suggestion to mount the scope perpendicular to the bore of the rifle? I have never understood how you can position anything in a relationship perpendicular to a round object. Since it is round, it really has no X or Y axis does it? Level is level, and plumb is plumb, right? If a cylinder is rotatated, up and down don't rotate with it, they are fixed in space. The bullet still exits the barrel, and gravity still pulls straight down on it. Any wind still deflects it to the same degree.

SNIP
How about the Leaning Tower of Pisa ? Buildings are built perpendicular to the round Earth so that all gravitational forces keep it from falling over.

Think of the reticle (extended) as either passing through the center of the bore or it doesn't! The bullet will exit the bore and fall toward the center of the Earth. It make no difference which way the rifle is pointed, gravity pulls it in a straight line ( in a vacuum).

You also are not aiming with the bore, but instead you are using an artificial aiming point some distance away.
You are sighting on a line that is in-line with the vertical reticle line and not the bore. If the scope reticle is at "some angle" NOT perpendicular, then as you dial up/down you are following a path that is Diagonal to the Bore. There will be a point the the two cross, and then the bullet would fall exactly in line with your intended shot, but you would need to always shoot at that distance to ensure a perfect shot...or compensate left/right for every other shot.
Also if you tried to "dial up/down", your click distance would not equal exactly that of the scope.

edge.
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  #25  
Old 08-31-2008, 05:31 PM
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Re: Reticle Perpendicularity by Darrell Holland

Edge -

I appreciate your response, and maybe I am just too blunt an instrument to get this. However, I am still confused. My grandpa always told me not to throw pearls before the swine, and in this case perhaps I am the swine...!

As I read your post, I was thinking that we were (at least partly) saying the same thing. We both agree that having the reticle perpindicular (or parallel to a plumb line) when aiming is what is important because gravity pulls straight down on the bullet after it exits the rifle. This would happen if the gun were held a 90 degrees, upside down, or any angle in between. My thought is that since my natural hold is a canted one, I should have the scope rotated so that with that natural hold the reticle's axes are plumb and level respectively.

If I were to mount the scope as suggested by the article, when I held the gun the axes would not be plumb or level, and as I dialed in my clicks for extended range the bullet's impact would begin to wander off diagonally. To avoid this, I would have to "muscle" the rifle into an unnatural hold so that the reticle would remain perpindicular.

I guess my point is that it seems it is only important that the scope's reticle is oriented perpindicular and not the rifle as well. An extreme example of this would be that you could lay the rifle on its side, rotate your scope 90 degrees, obtain a zero, and still make hits at distance because the vertical wire of the reticle would be running parallel to gravity's pull.

Similarly, if the rifle's scope were mounted at a precise 90 degrees to a perfectly machined and fitted scope base, the reticle would only remain perpindicular if the shooter held it that way, or if it were in a solid and well-engineered rest that was leveled. If holding a rifle like that from a field shooting position is a comfortable hold that provides a good natural point of aim, it would make sense. I just can't seem to shoot that way.

Again, to summarize my (possibly very misguided) point of view: It seems that what is important is to be sure the reticle's vertical axis is as close to parallel with gravity's pull as is humanly possible prior to lighting the torch on your favorite long range pill. It doesn't seem, to me, to matter if the rifle is also perfectly oriented that way because once the bullet exits the barrel gravity pulls straight down on it. So we would have the bullets path running parallel to the vertical axis of the reticle. By the way, I tested this to 300 yards yesterday with my current setup. I know, not exactly long range, but the best I had access to from a bench, and no noticeable drift was observed.

For those of you who still think I am full of beans, please don't get too frustrated with me, and try to help a dense rifle enthusiast out. I'd really like to be convinced I am wrong, if I indeed am. Of course, maybe I don't need to understand, I should just listen and move on. Danged German-Irish blood...
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  #26  
Old 08-31-2008, 10:33 PM
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Re: Reticle Perpendicularity by Darrell Holland

mkollman

You are entirely correct. It does not matter whether your rifle is canted or uncanted as long as you shoot with your reticle horizontal and vertical.

Reference:
Practical Riflery Forums... techniques and equipment of the practical rifleman... :: View topic - Canted scope or canted rifle?
Cant Errors - Long range shooting
The Optics Talk Forums: cant revisited
The Optics Talk Forums: Rifle cant, scope cant and counting horses teeth
The Optics Talk Forums: Scope reticle leveling / rifle cant
The Optics Talk Forums: Rotation or Cant problem?

If you are shooting at longer ranges where it matters then the trick is to shoot with your reticle level. IMO having an anti-cant device is helpful there.
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  #27  
Old 08-31-2008, 11:02 PM
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Do 2 wrongs make a right? (canted scope and canted rifle)

Mkollman, Woods,

Quote:
Originally Posted by woods View Post
mkollman

You are entirely correct. It does not matter whether your rifle is canted or uncanted as long as you shoot with your reticle horizontal and vertical.

...

Actually, it makes a 'little' bit of difference. Let's say I have a rifle that is setup with the reticle canted a bit because I always hold the rifle a little canted, 'do 2 wrongs make a right?'.

If I sight the rifle in for 100yds and shoot at zero clicks, 10MOA up and 10MOA down at 100yds, I will see bullet holes in the bullseye and 10MOA high and low of the bullseye. But there is more to the story.

Since I am canting the rifle, I'm moving the center line of the scope away from the center line of the bore (bullet path); for the sake of argument lets say the actual crosshair is 1/2" to the left of the bore of the rifle when I shoot.

The bullet will move from right to left (in relation to the line of the scope) exactly 1/2" from the time it leaves the muzzle until it strikes the bullseye at 100yds. When I decide to shoot 1000yds, I'll crank my scope up the right number of MOA for 1000yds and fire away. I will actually hit to the left of the bullseye 4 1/2" at 1000yds, because the line of the scope and the flight of the bullet intersect at 100yds and get further apart at longer distances.

If the centerline of your scope and the bore are only 1/10" apart, you will probably never see the difference, but it will be there.

I hope that all made sense.

AJ
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  #28  
Old 09-23-2008, 10:35 PM
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Re: Reticle Perpendicularity by Darrell Holland

Gentlemen,


Greetings, I'm back from our trip and thougth a few words may be interest to those interested in point of impact change when we cant the rifle at long range.

For this test I used a 6mm XC with 107 SMK bullets. The rifle was zeroed at 100 yds and 6.75 MOA was dialed into the scope to shoot at 385 yds. The rifle was canted left and right 5 10 and 20 for this test. At 385 yds. the bullet impact was 4.7 inches left for a 5 degree cant and 9.0 inches for a 10 degree cant ( left hand cant) bullet impact was .7-.75 inches high as well. On a right hand cant the bullet was 3.6 inches right + 1.4" high, 10 degrees was 6.6 inches right and +1.1 inches high.

With a left hand cant of 20 degrees bullet impact shifted 15.5 inches left and was 2.6 inches low.

Keep in mind that we are changing the axis of the bore relative to the scope if it is not perpendicular. Anytime we cant the rifle slightly ( left or right) the bore moves up from BDC ( bottom dead center) causing a slighlty higher point of impact. When cants beyond 15-20 degrees ( this is a HUGE cant and would problably never occur in the field) the bullet impact is low.

Shooting and testing your rifle is the best way to convince yourself of this phenomenon, yes, sight height above bore does matter. Our test rifle was 1.75 inches above the bore.

Visualize two tubes mounted one above the other with a cross hair running thru the centerline of each tube. Now take the cross hair on the top tube and cant it 10 degrees left or right. Next, take the cant out ( making the reticle plumb) and watch the bore ( the bottom tube) move left or right changing the point of impact. Due to angular deflection the amount of error increases with the range.

Its imparative that the scope be mounted perpendicular to the bore for best results at long range. Anytime we cant the reticle relative to the bore we lose the ability for a true vertical path from our zero and sight alignment. With a canted reticle we induce left/right axis as well as gravity ( vertical drop ) and the rifle will need windage and elevation to shoot past our initial zero.

Hopefully this helps clear some of the muddy water concerning reticle perpendicularity.

As always be safe and shoot straight!

Respectfully,


Darrell Holland

It was interesting to note that groups canted to the right were smaller than those to the left??? The cant and bullet/barrel harmonics may be responsible for this??? Groups to the right were .830 and 1.1 inches in size. Groups to the left ran 1.3-1.5 inches... Further testing may provide answers???

It would be nice to see members duplicate the above test under near perfect conditions and let me know their results??/ I would be most interested to hear from you.
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