georgehwbush
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the more out of focus the parallax is, and the larger the distance to target; the longer the cutting blades of the scissors are.
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Physics of shooting a rifle
- Thread starter tangolima
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In my experience this is normal unless you are accustomed to training your brain to sight in with both eyes open.
Even then you will have a dominant eye. Been shooting both eyes for most of Army Career as do the vast majority. That's what the way we trained. You still have a dominant eye and I was always a "handicapped" shooter with left eye dominance while right handed. Years ago, the Army made you shoot left handed if your were left eye dominant. We found it makes little difference. I leave the same fist sized holes in the paper at qualification time shooting 3 mags no matter which eye I rely upon and get the same score, lol.
I could be wrong as optics is not my forte. But I don't think parallax itself causes any error directly. With zero angle A, there will be no poi shift, however much is the parallax.
Consciously keeping the shooter's eye coaxial with the optics works. The black ring seen around the image in the scope must be even and symmetrical. It works but could be restrictive for certain applications. Some optics designs try to help relax such restrictions.
Angle A is the key. It depends on the shift of the shooter's eye position of course. But it is also function of the distance of the reticle to the shooter's eye. Traditional telescopic sight is not very good as the reticle is rather close to the shooter's eye, 6" or so. Reflex sight (red dot) creates the reticle (the dot) at infinite distance, making it much more tolerant to change of shooter's eye position.
Talking about parallax, it is a common understanding that it worsens with target distance. It is not totally true I'm afraid.
Let's say a scope has fixed parallax setting for 100yd (3600") and its objective lens has focal length of 6". Using the lens equation 1/f=1/u+1/v, we have the following table
u, v, parallax
10yd, 6.102", 0.092"
25yd, 6.040", 0.03"
50yd, 6.020", 0.01"
100yd, 6.010", 0"
200yd, 6.005", -0.005"
500yd, 6.002, -0.008"
1000yd, 6.001", -0.009"
The position of the target image relative to the reticle changes side at 100yd. The parallax will never grow beyond 0.01" afterwards. Parallax is more of an issue for close distances.
-TL
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Consciously keeping the shooter's eye coaxial with the optics works. The black ring seen around the image in the scope must be even and symmetrical. It works but could be restrictive for certain applications. Some optics designs try to help relax such restrictions.
Angle A is the key. It depends on the shift of the shooter's eye position of course. But it is also function of the distance of the reticle to the shooter's eye. Traditional telescopic sight is not very good as the reticle is rather close to the shooter's eye, 6" or so. Reflex sight (red dot) creates the reticle (the dot) at infinite distance, making it much more tolerant to change of shooter's eye position.
Talking about parallax, it is a common understanding that it worsens with target distance. It is not totally true I'm afraid.
Let's say a scope has fixed parallax setting for 100yd (3600") and its objective lens has focal length of 6". Using the lens equation 1/f=1/u+1/v, we have the following table
u, v, parallax
10yd, 6.102", 0.092"
25yd, 6.040", 0.03"
50yd, 6.020", 0.01"
100yd, 6.010", 0"
200yd, 6.005", -0.005"
500yd, 6.002, -0.008"
1000yd, 6.001", -0.009"
The position of the target image relative to the reticle changes side at 100yd. The parallax will never grow beyond 0.01" afterwards. Parallax is more of an issue for close distances.
-TL
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But I don't think parallax itself causes any error directly.
It directly causes a shift in the POA. It does not effect ballistics but starts the whole process off on the wrong scale.
Parallax is more of an issue for close distances.
Depends on the optics. Distance comes with a larger displacement so it does matter. A small angle makes for a larger displacement the longer the distance.
Yes, it is a big deal for red dot sights at close distances as all red dots have parallax due to their physics. That is why stock weld and head position are critical as well as one of the major advantages of holographic sights.
https://www.greeneyetactical.com/2017/07/27/comparative-study-of-red-dot-sight-parallax/
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georgehwbush
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well imperical testing personally my 10 42mm scope with the parallax adjustment at 100yds, i can move around behind the scope without changing poi/poa, but if i turn and aim at something say 500yds out, and run the same test the target verses retical moves around about 6 to 8 inches. and it gets worse at longer ranges. and if i adjust out the parallax at 500yds then aim at a 100yd target and run the same test it moves around about 1.5 inches.
that's all i know, just what i see in the scope.
that's all i know, just what i see in the scope.
georgehwbush,
Parallax was a big deal in long distance shooting as a principle. In the Army, we were taught to always check for it before a long distance shot. It could make the difference between hitting the target and a miss. Your mileage may vary depending on the Optic. I will say Leupold Mk3 10x42 and Mk4 10x40 fixed power scopes are considered a pretty good scopes and the ones I used for most of my long distance experience.
Lots of Red Dot's claim to be parallax free. We found that not to be the case and that is backed up by the physics of a Red Dot Optic. Emphasis was on maintaining the same stock weld just shooting with a Scope even in unusual shooting stances.
"Like a Turret" was a phrase often repeated in our training.
Parallax was a big deal in long distance shooting as a principle. In the Army, we were taught to always check for it before a long distance shot. It could make the difference between hitting the target and a miss. Your mileage may vary depending on the Optic. I will say Leupold Mk3 10x42 and Mk4 10x40 fixed power scopes are considered a pretty good scopes and the ones I used for most of my long distance experience.
Lots of Red Dot's claim to be parallax free. We found that not to be the case and that is backed up by the physics of a Red Dot Optic. Emphasis was on maintaining the same stock weld just shooting with a Scope even in unusual shooting stances.
"Like a Turret" was a phrase often repeated in our training.
Note: Parallax indexes aren't always accurately-marked on optics.
Move your head from side to side while looking through scope at target at the desired distance. Adjust to minimize movements by adjusting the objective lens while scope is mounted on a sturdy rest (like a rifle in a good vise) or the scope in a stiff-padded vise, but not so stiff as to deform the scope.
Move your head from side to side while looking through scope at target at the desired distance. Adjust to minimize movements by adjusting the objective lens while scope is mounted on a sturdy rest (like a rifle in a good vise) or the scope in a stiff-padded vise, but not so stiff as to deform the scope.
I have been thinking about this on and off, clsee to quantitative analysis but quite there yet. Qualitatively I have come to a few conclusions.
1. Parallax (distance between target image and reticle) changes sight picture. When the shooter's eye deviates from the optics' axis, it appears as POA off the target. If the shooter's eye is coaxial to the optics, there is no error, other than the blurriness of the target image.
2. The changes in sight picture becomes more noticeable with increased parallax.
3. Parallax has upper bound for distant target. It is theoretically boundless for close target.
4. The change in sight picture prompts the shooter to "correct" his aim erroneously, so as to cause error in poi. However error in poi is independent of the amount of parallax. But rather it depends on how much his eye is off axis.
-TL
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1. Parallax (distance between target image and reticle) changes sight picture. When the shooter's eye deviates from the optics' axis, it appears as POA off the target. If the shooter's eye is coaxial to the optics, there is no error, other than the blurriness of the target image.
2. The changes in sight picture becomes more noticeable with increased parallax.
3. Parallax has upper bound for distant target. It is theoretically boundless for close target.
4. The change in sight picture prompts the shooter to "correct" his aim erroneously, so as to cause error in poi. However error in poi is independent of the amount of parallax. But rather it depends on how much his eye is off axis.
-TL
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Looks like unclenick will need more time before he comes back with illustrations. I'm going ahead to fill the rabbit I dug on bullet stabilization.
To recap. A bullet, except a perfect round ball, has center of pressure (CP) ahead of center of gravity (CG), so it is inherently unstable. It will tumble in flight without gyroscopic stabilization. All stabilized projectiles have the ability to turn into the wind. For instance, when flying in a cross wind from the right, the feather in an arrow's tail gets push to the left, and the arrow head points into the apparent wind to the right, so as to minimize the cross wind effects. When it comes to a bullet, the mechanism becomes more complicated. Obviously it has much to do with spinning of the projectile. But exactly how?
It has been discussed in any books on external ballistics. Unfortunately getting a simple and intuitive picture of the mechanism hasn't been trivial. I tried to understand it with the precessional property of a gyroscope.
The bullet has right hand twist and cross wind is again from right to left. The wind acts on the bullet's CP, imposing a torque to turn the bullet nose downwind to the left. With precession, the bullet nose will NOT turn to left, but it will turn upward instead. With this nose-up attitude, the wind acts on the CP to impose a torque trying to turn the bullet nose further up. Again due to gyroscopic precession, the bullet nose ended up pointing to the right, into the wind. This closes the negative feedback loop and the bullet assumes a stable nose-up-right attitude traveling down range.
It sounds like a 2 steps process, up and to the right. However it doesn't snap into the final state immediately. Instead the bullet nose oscillates up/down and left/right for short period of time, during which the bullet nose appears moving in an elliptical trajectory till it settles down.
Cross wind right to left, POI high-left. Cross wind left to right, POI low-right. No tumbling if the twist rate is adequate.
-TL
Sent from my SM-N960U using Tapatalk
To recap. A bullet, except a perfect round ball, has center of pressure (CP) ahead of center of gravity (CG), so it is inherently unstable. It will tumble in flight without gyroscopic stabilization. All stabilized projectiles have the ability to turn into the wind. For instance, when flying in a cross wind from the right, the feather in an arrow's tail gets push to the left, and the arrow head points into the apparent wind to the right, so as to minimize the cross wind effects. When it comes to a bullet, the mechanism becomes more complicated. Obviously it has much to do with spinning of the projectile. But exactly how?
It has been discussed in any books on external ballistics. Unfortunately getting a simple and intuitive picture of the mechanism hasn't been trivial. I tried to understand it with the precessional property of a gyroscope.
The bullet has right hand twist and cross wind is again from right to left. The wind acts on the bullet's CP, imposing a torque to turn the bullet nose downwind to the left. With precession, the bullet nose will NOT turn to left, but it will turn upward instead. With this nose-up attitude, the wind acts on the CP to impose a torque trying to turn the bullet nose further up. Again due to gyroscopic precession, the bullet nose ended up pointing to the right, into the wind. This closes the negative feedback loop and the bullet assumes a stable nose-up-right attitude traveling down range.
It sounds like a 2 steps process, up and to the right. However it doesn't snap into the final state immediately. Instead the bullet nose oscillates up/down and left/right for short period of time, during which the bullet nose appears moving in an elliptical trajectory till it settles down.
Cross wind right to left, POI high-left. Cross wind left to right, POI low-right. No tumbling if the twist rate is adequate.
-TL
Sent from my SM-N960U using Tapatalk
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stagpanther
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Arrows are also spin-stabilized.
Spin can be added to arrow to supplement, by angling the feather. Perhaps it is for short arrow with very small fins, such as the one for cross bow. But it is hard to imagine a solely spin stabilized arrow.
Airgun pellet is another example of spin supplemented fin stabilization. Like a badminton shuttle cork, the pellet is stable without the spin. The spin is needed when it exits the muzzle, where airspeed is negative. It is another rabbit hole to go down.
-TL
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Airgun pellet is another example of spin supplemented fin stabilization. Like a badminton shuttle cork, the pellet is stable without the spin. The spin is needed when it exits the muzzle, where airspeed is negative. It is another rabbit hole to go down.
-TL
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georgehwbush
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44AMP (I guess this is one of those physics problems I just don't get. Like the one where a guy gets into the pool, swims to the far end, swims back and climbs out where he went in.
The instructions for that one said to use physics to prove he went nowhere.
I don't think he went nowhere. )
it's quite obviously that "he" did NOT "not go anywhere" the rotational velocity of the planet moved him along at about mock one. "depending on latitude" And the orbital velocity of the planet moved him along at around 60,000 mph. not to mention the movement of the solar system itself... we are indeed traveling, and i feel that the math should reflect that; otherwise it's only partial. 2 + 2 = 4 but only if there are only two factors involved else it is 2+2+$Variables=answer?
The instructions for that one said to use physics to prove he went nowhere.
I don't think he went nowhere. )
it's quite obviously that "he" did NOT "not go anywhere" the rotational velocity of the planet moved him along at about mock one. "depending on latitude" And the orbital velocity of the planet moved him along at around 60,000 mph. not to mention the movement of the solar system itself... we are indeed traveling, and i feel that the math should reflect that; otherwise it's only partial. 2 + 2 = 4 but only if there are only two factors involved else it is 2+2+$Variables=answer?
stagpanther
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Unless the shooter wants an arrow optimized for missing the target--they are all spin-stabilized. The shafts will warp upon launch and without some means of stabilization they will wobble to the target.Spin can be added to arrow to supplement, by angling the feather. Perhaps it is for short arrow with very small fins, such as the one for cross bow. But it is hard to imagine a solely spin stabilized arrow.
All stabilized projectiles have the ability to turn into the wind. For instance,
Yes....
Using the GROUND as a frame of reference.
Using the airmass, they adjust to move with the airmass and become a part of it.
Hmm.. arrow fletching is rabbit hole I'm not quite ready to go down just yet. The most basic form is straight fletch, with no spinning. Arrow flies stable and true. So arrow is fin stabilized, i.e. it doesn't tumble in flight. Offset and helical fletches are the newer and more advanced forms that introduce different degrees of spinning. They certainly make things better for certain applications.Unless the shooter wants an arrow optimized for missing the target--they are all spin-stabilized. The shafts will warp upon launch and without some means of stabilization they will wobble to the target.
-TL
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stagpanther
New member
Most arrows will have a head that keeps the CG at the front--so yeah; there will be more resistance to loss of control i.e. tumbling, in that sense you could launch one without any fletches at all being generally fairly long you don't need much of a spin since it's going fairly slow compared to a bullet. I used to fletch my own arrows for primitive archery; I could definitely tell an accuracy/consistency difference between arrows that had a spin-inducing offset (I think most jigs are 3 or 4 % IIRC) and ones that didn't--I fletched a few with no offset.Hmm.. arrow fletching is rabbit hole I'm not quite ready to go down just yet. The most basic form is straight fletch, with no spinning. Arrow flies stable and true. So arrow is fin stabilized, i.e. it doesn't tumble in flight. Offset and helical fletches are the newer and more advanced forms that introduce different degrees of spinning. They certainly make things better for certain applications.
No doubt proper amount of spin makes quite a bit of difference. Archery is something I haven't done enough. Did backyard bow and arrow with kids (byproduct of watching hunger games) and that's about it. Straight fletch of course.Most arrows will have a head that keeps the CG at the front--so yeah; there will be more resistance to loss of control i.e. tumbling, in that sense you could launch one without any fletches at all being generally fairly long you don't need much of a spin since it's going fairly slow compared to a bullet. I used to fletch my own arrows for primitive archery; I could definitely tell an accuracy/consistency difference between arrows that had a spin-inducing offset (I think most jigs are 3 or 4 % IIRC) and ones that didn't--I fletched a few with no offset.
Bullet spin stabilization is rather different. Actually I have found almost anything rotational is quirky (unpredictable). It is more visible when shooting "suboptimal" bullets; 22lr over 100yd and airgun pellet for example. They have been my poor man's training methods for long range shooting. Now I will add / subtract elevation when holding for cross wind.
-TL
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You're both right. Arrows are stable without spin (they won't tumble without spin) which means they are aerodynamically stabilized--the forces of drag stabilize the arrow without the need for spin. But a little bit of spin helps eliminate issues related to minor variations in the arrows that would otherwise cause them to veer off target.
The spin is nowhere near what it would take to stabilize the arrow if it were truly unstable, but it is enough to make it fly more true by "distributing" the aerodynamic flaws so that they can't cause the arrow to veer in one direction as would happen if the arrow wasn't spinning at all.
The spin is nowhere near what it would take to stabilize the arrow if it were truly unstable, but it is enough to make it fly more true by "distributing" the aerodynamic flaws so that they can't cause the arrow to veer in one direction as would happen if the arrow wasn't spinning at all.
If they fly for a very long time (relatively speaking), they will become part of the air mass. In practice, that's not going to happen with bullets because the time they are in the air is not sufficient for them to acquire the same sideways velocity as the air mass.Using the airmass, they adjust to move with the airmass and become a part of it.
Lest we jump back to the same heated debate we went through not long ago, let's jump into another more productive rabbit hole, shall we?
It is actually an discussion I was involved on other forum. It is about spring poundage. Say my AR has a 16 lb recoil spring. What exactly does 16lb mean? In physics a spring's Hook's constant specifies its stiffness. How are poundage and Hook's constant related?
-TL
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It is actually an discussion I was involved on other forum. It is about spring poundage. Say my AR has a 16 lb recoil spring. What exactly does 16lb mean? In physics a spring's Hook's constant specifies its stiffness. How are poundage and Hook's constant related?
-TL
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If they fly for a very long time (relatively speaking), they will become part of the air mass. In practice, that's not going to happen with bullets because the time they are in the air is not sufficient for them to acquire the same sideways velocity as the air mass.
____________
I will go with what Sierra Engineers and the Laws of Aerodynamics relate.
https://youtu.be/FIPhwp-V2RQ
Because of the initial balancing of the forces as the bullet picks its angle of repose and the fact the math for bullet ballistics is solving the ground frame of reference, you seem to think that negates the Laws of Aerodynamics. It does not.
You are free to disagree
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