Point of aim vs point of impact

[DShortt]

To an extent it's the Indian, not the arrow. Unless you have a bent arrow. The inherent dispersion of your rifle and cartridge cannot be overcome with skill, only luck part of the time. It's always good to have a fellow shooter you know is much better than you shoot the rifle, and have him watch you shoot it.
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I agree. Unfortunately at this time while I know several other guys who shoot the majority of them are either hunters or pistol shooters and so they aren't interested in bench shooting apart from sighting in a rifle occasionally. I plan to join a gun club in my area soon and I'm hoping I'll meet some people there who can help me out.

 

[Brians356]

I've said right along that consistent cant has no deleterious effect on group size. But I need to work out for myself that distance between POA and POI has zero effect on group size assuming significantly inconsistent cant creeps in. Not saying you're wrong, in fact hoping you're right!
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Turns out I was right. The steeper the bullet's departure angle of above line-of-sight to POA, the larger the horizontal displacement caused by any cant. If the cant is consistent shot-to-shot, the horizontal displacement will also be consistent, and not affect group size. But any variation in cant will produce a larger variation in horizontal displacement. And there's always some variation in cant, however small. The overall effect may well be negligible, but it's true that arranging the POI closer to POA (shortening distance PT in the diagram below) will to some extent reduce cant variation effects on group size.

The formula for horizontal displacement from cant is: TT′ = PT sine C

An NRA Shooting Sports Journal | Effects of Cant

Understanding rifle cant includes understanding physics, but the most errors come from not holding the rifle bore axis and scope axis on a vertical plane

www.ssusa.org

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[BobbyJ]

I'm a casual shooter and have been having troubles trying to improve my shooting and group sizes. I started a thread yesterday about anti-cant devices and one of the replies jarred something and I believe I've had a moment of clarity. I purposely do not shoot my point of aim so as to not loose it but I've never paid attention to how far the point of impact is from the point of aim. It's just occurred to me that it seems that the further the point of impact is from the point of aim then mistakes such as scope/rifle cant will be amplified. Is this plausible or no? I'd just go to the range and see for myself but as I'm still a slave to the grind that's not possible for a few days so I thought I'd see what some of you think. I'm betting several here know the exact answer so I'm looking forward to your replies. Thanks for looking.

I think people are getting confused but what you are asking. Which to me is
1. Does it matter if the POA and POI are the same spot?
2. If they differ does canting my rifle worsen it vs POI and POA being at the same point?

1. Yes it can matter
2. Yes it can make it worse.

Let me give you one example and take it to and extreme. Lets say you have POA at the bullseye but all your rounds are hitting 100 feet to the left.

Now you rotate the gun but keeping the cross hairs on the center of the Bullseye. For each degree you rotated the gun the error is going to be much higher (Think of a 100 foot clock hand) then if your POI and POA are at the same spot (Bullseye). Your scope is not mounted inside the bore so there is a still some error due to the scopes being mounted above the bore. But it will be less per degree of can't vs the 100 foot clock.

For each tick of the clock the angle is the same but the linear distance per tic is going to be much higher than for example a wrist watch.

Bottom line - Don't can't your rifle use a bubble level.

- Reticle should bisect the barrel (it does not always depending on how the scope bases, barrel etc are made. So even a level scope and receiver could be offset where the reticle never bisects the bore. It can be verified using a mirror test.

 

[Brians356]

I am right there with you. I bought a scope mounted anti-cant device so that I can check to see the effects of cant myself.

Unless you have a setup that will shoot very tiny groups consistently, you're not going to be able to demonstrate positively the effect of cant (which is a fact anyway) and you'll be wasting considerable ammunition in trying. Just level targets, align reticle with targets when aiming, and apply your time and money more productively elsewhere. Eventually, there awaits you learning to dope wind!
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[Will Henry]

Having the group form distant from the point of aim has no influence on the dispersion caused by cant.
A target consisting of a piece of paper with a dot in the middle is perfect. As long as the scope reticle is aligned with the piece of paper, cant (or lack thereof) will be consistent and group size unaffected. WH

 

[Brians356]

Having the group form distant from the point of aim has no influence on the dispersion caused by cant.

From the NRA article I liked to:

"We see that the magnitude of the effect is dependent on both the angle of elevation (which governs the height PT) and the angle through which the rifle is canted."

Increasing the verticle distance between POI and POA (PT) obviously increases angle of elevation. Therefore it increases the magnitude of the effect of cant. Can you cite a formula for the effect of cant that refutes this?
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[BobbyJ]

Turns out I was right. The steeper the bullet's departure angle of above line-of-sight to POA, the larger the horizontal displacement caused by any cant. If the cant is consistent shot-to-shot, the horizontal displacement will also be consistent, and not affect group size. But any variation in cant will produce a larger variation in horizontal displacement. And there's always some variation in cant, however small. The overall effect may well be negligible, but it's true that arranging the POI closer to POA (shortening distance PT in the diagram below) will to some extent reduce cant variation effects on group size.

The formula for horizontal displacement from cant is: TT′ = PT sine C

View attachment 1372328

An NRA Shooting Sports Journal | Effects of Cant

Understanding rifle cant includes understanding physics, but the most errors come from not holding the rifle bore axis and scope axis on a vertical plane

www.ssusa.org

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I never doubted you

 

[Brians356]

I never doubted you

To be fair, a few inches increase of PT at 100 yds for a sporting cartridge fired with a degree or two of cant will have a negligible effect on observable displacement.
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[DShortt]

Turns out I was right. The steeper the bullet's departure angle of above line-of-sight to POA, the larger the horizontal displacement caused by any cant. If the cant is consistent shot-to-shot, the horizontal displacement will also be consistent, and not affect group size. But any variation in cant will produce a larger variation in horizontal displacement. And there's always some variation in cant, however small. The overall effect may well be negligible, but it's true that arranging the POI closer to POA (shortening distance PT in the diagram below) will to some extent reduce cant variation effects on group size.

The formula for horizontal displacement from cant is: TT′ = PT sine C

View attachment 1372328

An NRA Shooting Sports Journal | Effects of Cant

Understanding rifle cant includes understanding physics, but the most errors come from not holding the rifle bore axis and scope axis on a vertical plane

www.ssusa.org

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This is what I had pictured in my mind so I'm glad to see it demonstrated and particularly in a respected publication.
The down side of that is the target dot and 9x scope I've been using make it next to impossible to monitor cant to within a couple of degrees. I'd guess it's been 5 degrees +/-. Add to that the POA and POI being apart several inches. I've no doubt this is at least some of what I've been chasing for a long time.
As I've heard it said before 'it doesn't matter until it matters'.

 

[DShortt]

Having the group form distant from the point of aim has no influence on the dispersion caused by cant.
A target consisting of a piece of paper with a dot in the middle is perfect. As long as the scope reticle is aligned with the piece of paper, cant (or lack thereof) will be consistent and group size unaffected. WH

I completely agree provided that the reticle remains perfectly aligned. My mistake has been that I have not included a method to make certain of that perfect alignment. If I can't see it then I can't correct for it.
Pun noted.

 

[BobbyJ]

To be fair, a few inches increase of PT at 100 yds for a sporting cartridge fired with a degree or two of cant will have a negligible effect on observable displacement.
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Oh I know this but this forum is full of OCD shooters and its best to understand all the variables in order to make decisions. Many people over think and many under think it.

Even canted as long as you hold the gun the same way each shot groups are going to be the same.

Groups would be affected if the shooter has it tilted 5 degree right then 5 deg left for every shot. Not likely. More likely is zeroing with no can't then shooting at live game using a tree which causes a can't of 10 degrees counterclockwise in order to make that shot. Going to hit low and left.

 

[jelenko]

Deleted because my statement was incorrect.

 

[Brians356]

Groups would be affected if the shooter has it tilted 5 degree right then 5 deg left for every shot. Not likely. More likely is zeroing with no can't then shooting at live game using a tree which causes a can't of 10 degrees counterclockwise in order to make that shot. Going to hit low and left.

In the article an example says a 270 Win zeroed at 300 yards, canted 5 degrees, would see a horizontal displacement of about 2.5 inches. But 5 degrees is quite a bit in a benchrest scenario. If it was "dialed in" unintentionally in a firm setup, and thus repeated shot-to-shot, probably not noticed. Hunting big game, from an awkward position, a cant of 2 or 3 times that would likely end as one to remember fondly.
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[Brians356]

Well, yes. For 2 MOA [i.e., ~ 2"] at 100 yards sine c = ~.0006. So, the horizontal displacement due to a 2 MOA elevation difference at 100 yards is 2"X.0006 = .0012 inches. That's 1.2 thousandths of an inch.

Without checking your work, I say fair enough, since I allowed several times it's negligible in the context of the OPs scenario. Nevertheless the assertion that elevating POI "has no [as in zero] influence on the dispersion caused by cant" is obviously false. Academic, I grant you. But of such trifles, bar bets are made. ;-)

Edit: In your work, you don't state the assumed value of C (cant in degrees). So "sine C" is undefined. Over.
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[jelenko]

Hmm. First, please ignore my comments on the effect of cant. After reading the article, the effect is different than what I had in mind.

 

[Brians356]

Hmm. First, please ignore my comments on the effect of cant. After reading the article, the effect is different than what I had in mind.

Hey, I'm here to learn, thanks for prodding me to dig into this aspect.
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[Will Henry]

From the NRA article I liked to:

"We see that the magnitude of the effect is dependent on both the angle of elevation (which governs the height PT) and the angle through which the rifle is canted."

Increasing the verticle distance between POI and POA (PT) obviously increases angle of elevation. Therefore it increases the magnitude of the effect of cant. Can you cite a formula for the effect of cant that refutes this?
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I cannot cite a formula which refutes this; simply because I was incorrect in my statement. Not the part about the target, however! WH

 

[Brians356]

I cannot cite a formula which refutes this; simply because I was incorrect in my statement. Not the part about the target, however! WH

I agree. What's important is the consistency of setup, minimizing variation shot-to-shot of everything, including cant. I trust a visual reference seen through the scope, referenced against the reticle, more than I trust an external bubble level or the like. When available - the OP has a dot reticle.
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[BobbyJ]

I agree. What's important is the consistency of setup, minimizing variation shot-to-shot of everything, including cant. I trust a visual reference seen through the scope, referenced against the reticle, more than I trust an external bubble level or the like. When available - the OP has a dot reticle.
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I kind of like this can't discussion. For what its worth here is real world can't effect at 50 yards of a pellet guns shooting around 900 fps. I don't know the exact drop but its around 6" in chair gun calculator.
====================================
- Rifle is FX Bobcat .25 mk2
- Center line of bore to center line of scope measures aprox. 2.5"
- JSB Exact Kings .25 @ 25.4gr
- Range from rifle to target is 50 yards
- drop at 50 yards 6"
- velocity 900ish
======================================
Calculation:
Horizontal POI Error= Drop x sine(Cant Angle)
==================================
10 degree
6" drop at 50 yards
6 x sine(10)
6 x 0.174 =
= 1.04" (Real life measured .75 to 1.25")
===========================================
20 degree
6" x sine(20)
6" x 0.35
= 2.1 inches (Real life measured 1.5 to 1.70")
================================================


Due lower velocity and BC air guns suffer more with canting.
At 600 meters the cant sensitivity of a high-power rifle is of the order of 50 mm
per degree. Assuming a cant uncertainty of +/- 1 degree, this results in a total uncertainty
of about 100 mm or 4 inches.

 

[expiper]

I think people are getting confused but what you are asking. Which to me is
1. Does it matter if the POA and POI are the same spot?
2. If they differ does canting my rifle worsen it vs POI and POA being at the same point?

1. Yes it can matter
2. Yes it can make it worse.

Let me give you one example and take it to and extreme. Lets say you have POA at the bullseye but all your rounds are hitting 100 feet to the left.

Now you rotate the gun but keeping the cross hairs on the center of the Bullseye. For each degree you rotated the gun the error is going to be much higher (Think of a 100 foot clock hand) then if your POI and POA are at the same spot (Bullseye). Your scope is not mounted inside the bore so there is a still some error due to the scopes being mounted above the bore. But it will be less per degree of can't vs the 100 foot clock.

For each tick of the clock the angle is the same but the linear distance per tic is going to be much higher than for example a wrist watch.

Bottom line - Don't can't your rifle use a bubble level.

- Reticle should bisect the barrel (it does not always depending on how the scope bases, barrel etc are made. So even a level scope and receiver could be offset where the reticle never bisects the bore. It can be verified using a mirror test.

please tell about the mirror test !!---how do you do it ???---thank you--Roger