Physics of shooting a rifle

[stagpanther]

To get a bullet to cross the line of sight at 5 yds--you'd need a pretty heavily canted rail I would think.

[tangolima]

The cheapy lpvo with 30mm tube on the AR had no problem doing it. 2.5" at 5yd is 50 moa, which is quite a bit indeed.

For the rem 700, 1.5" at 5yd is 30moa. The scope was an old redfield with 1" tube. No problem either.

No canted rail in either case. Probably the scopes were about to max out.

-TL

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[tangolima]

In order to bore sight a rifle that doesn't allow looking through the bore, mini 14 for instance, I have struggled with different methods; mirror, reflector etc.

Then it hits me. Why am I doing this? To get on paper, is it not? I just pull in the 24"x24" target really close, say 10yd. Fire one shot with sight right on the middle of the target. I will have to really screw up not to be on paper. Adjust turrets to put poi under poa by amount equal to sight height. Fire another shot to confirm. Push the target back to 100yd and fire again. It will be on paper.

That's how I bore sight without sighting through the bore. It always work. Why don't I do it for all guns? I could. But it involves calling the range cold to move the target. I only do that when I have to.

-TL

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[tangolima]

Let's try another light one in optics, shall we? It is actually based a discussion on other forum. There they were discussing shift in poi caused by phoria. The op had such medical condition that his eye ball shift slightly when the other eye was covered. He noticed significant poi shift when shooting with both eyes open or one eye closed.

Members were perplexed. Everyone scrambled to do his own test. I believe it is parallax. What think you?

Say it is a FFP scope with 100yd fixed parallax. The reticle in the tube is located 8" in front of the shooter's eye. The target is at 50yd, so there is parallaxing going on. If the shooter's eye shifts 0.01" to the left, which direction will the reticle appear to shift in his field of view? If he adjusts his aim to realign the reticle and target, the poi will shift. By how much?

-TL


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[georgehwbush]

both my ar's 10 and 15 are zeroed at 100yds and both will hit low at any shorter range (ANY SHORTER RANGE) so the 100yard mark is the first crossing of the trajectory and line of sight, that's from imperical testing not just calculations.

now it should be noted that the sights are 2.5 and 2.75 inchs above bore respectively. and the speed of projectile matters too.

[tangolima]

Originally Posted by georgehwbush

both my ar's 10 and 15 are zeroed at 100yds and both will hit low at any shorter range (ANY SHORTER RANGE) so the 100yard mark is the first crossing of the trajectory and line of sight, that's from imperical testing not just calculations.



now it should be noted that the sights are 2.5 and 2.75 inchs above bore respectively. and the speed of projectile matters too.

100yd is indeed near the apex that touches the line of sight. It is dictated by the tall sight height.

-TL

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[stagpanther]

Is this different from simple right/left eye dominance hard-wired into sighting especially for those who are accustomed to shutting the left eyelid (reverse if you are left dominant)?

[tangolima]

I'm not 100% sure, but it is a medical abnormality.

https://medical-dictionary.thefreedictionary.com/phoria

-TL

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[stagpanther]

In my experience this is normal unless you are accustomed to training your brain to sight in with both eyes open.

[tangolima]

I managed to train myself to shoot with both eyes open. The major difficulty to overcome is seeing 2 targets. Sometimes the "wrong target" is picked to put the sight on. With enough practice, somehow the target seen by the non-dominant eye disappears and replaced by peripheral awareness.

But I think phoria, what the op of the discussion was diagnosed of, is different. There isn't dual vision. The dominant eye just perceives the target differently when the non-dominant eye is covered. There wouldn't be a diagnosis if it was normal, I think.

-TL

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[georgehwbush]

that's why i choose to adjust out the parallax on my optics.

i have owned scopes that didn't have adjustment, and for speed, that's better if quick is more important than accurate.

for long range and small groups, accurate is more important than speed.

and yes i have forgotten to adjust parallax out and missed the mark. kinda gives one a case of jackass knoggen. but we get over that pretty quickly anyway.

at my age and poor vision, non-scoped percision shots are limited to about 50 yards. that's why all my long guns have scopes. my ar15 is the only thing that i run with irons + red-dot & flip to the side magnifier. but i have quick detach scope dedicated to it also, just not always on it.

[rickyrick]

I used to feel that parallax at close range didn’t matter so much… until I crossed paths with a particularly aggressive skunk; parallax got the better of me and the skunk won the matchup.

[tangolima]

It is a bit difficult to visualize, but here is my understanding.

The scope forms an image of the target at certain focal plane. When the focal plane coincides with the reticle, there is no parallax. Say the shooter's eye shifts to the left by a small distance. He still focuses on the reticle. His line of sight is now deviating from the scope's axis by a small angle A.

No problem with zero parallax. The reticle is always on target. With parallax, however, there is distance between the target image and reticle. With angle A, the image remains the same, but the reticle appears to move, the direction of which depends on the target distance. It moves with the eye when the distance is less than the scope's parallax setting, and the other way otherwise.

If the shooter chooses to realign the reticle with the target, the poi will be off. The amount in moa is the same as angle A, if the scope has 1x magnification. The error in moa actually decreases at higher magnification.

Now it is all about angle A. Parallax will become a non-issue if angle A is very small. But how?

-TL

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[georgehwbush]

you are back to the sizors; angle A is the amount the handles are apart, the pivot is the retical + parallax the cutting points are the point of aim and point of impact.

[georgehwbush]

i know it's kind of an over simplification; but the reflex angle is the whole point. sight picture vs bore axis A > & poi vs poa B <

[georgehwbush]

the more out of focus the parallax is, and the larger the distance to target; the longer the cutting blades of the scissors are.

[davidsog]

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.

[tangolima]

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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[davidsog]

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/201...ight-parallax/

[georgehwbush]

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.

[davidsog]

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.

[Picher]

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.