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Old November 21, 2019, 06:57 PM   #1
BoogieMan
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Point of impact versus POA- hunting.

I sight all my hunting rifles at 100yds. The hunting I do ranges from 20yds to 150yds+-. Several deer I have taken at close range I have noticed the POI being below the POA. I am shooting from an elevated stand. Is the issue caused by the elevation or by parallax? What is the proper way to compensate or fix this?
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Old November 21, 2019, 07:43 PM   #2
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Parallax.

Get an external ballistics program and chart the trajectory for your chosen ammunition. Then learn to use the markings on your scope's reticle to adjust your point of aim when you know you're shooting at distances other than 100 yards.
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Old November 21, 2019, 07:54 PM   #3
BoogieMan
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@aquila Blanca- new scope has parallax adjustment on side. 15yd to 100yd & beyond. So, yourconfident that the difference is because of parallax and not elevation?
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Old November 21, 2019, 07:57 PM   #4
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The scope sits above the barrel so the POI crosses the POA twice. The first time will be somewhere around 25 yds.
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Old November 21, 2019, 07:59 PM   #5
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Sight the rifle at 1-1/2" high at 100 yards and don't worry about it. Parallax shouldn't make enough difference to matter on deer size targets at close range. Shooting down hill usually means a rifle will shoot high, not low.
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Old November 21, 2019, 09:27 PM   #6
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Quote:
Originally Posted by BoogieMan
@aquila Blanca- new scope has parallax adjustment on side. 15yd to 100yd & beyond. So, yourconfident that the difference is because of parallax and not elevation?
"Elevation" doesn't have much to do with it ... directly. What happens is that, at closer distances, if your stand is high you have to shoot down at a fairly steep angle. Bullet drop is affected only by the horizontal distance, not the diagonal distance. The simplest example is a 45 degree angle, but that would make for very short shooting distances. You mentioned 20 yards, so let's go with that. To make the math easier, let's use a stand height of 30 feet, which (conveniently) is 10 yards.

So the geometry is a triangle. Assume flat, level ground. You are 10 yards straight up, and your target is 20 yards straight out. But your line of sight/aim is along the downward hypotenuse of the triangle. That distance is 22.36 yards. So if you used a rangefinder, it would tell you that the distance is 22.36 yards, but when you shoot the bullet drop will be based on a horizontal distance of 20 yards. In addition, because you're shooting on a downward angle, the speed at which the bullet drops will be the drop due to gravity PLUS the vertical component (vector) of the muzzle velocity.

Let's say the muzzle velocity is 3,000 feet per second. Shooting at a downward angle of 27 degrees off horizontal, the result is that the bullet is moving at a horizontal velocity of 2,830 fps, and it's moving downward at a vertical velocity of 1,341 fps ... when it leaves the muzzle. That vertical velocity increases due to the effects of gravity as the bullet travels.

When you shoot across level terrain, the bullet starts out basically horizontal, so there's no vertical factor at the muzzle, and the drop is due entirely to the effect of gravity.
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Old November 21, 2019, 10:24 PM   #7
Double Naught Spy
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Quote:
I sight all my hunting rifles at 100yds. The hunting I do ranges from 20yds to 150yds+-. Several deer I have taken at close range I have noticed the POI being below the POA. I am shooting from an elevated stand. Is the issue caused by the elevation or by parallax? What is the proper way to compensate or fix this?
How far below are we talking about?
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Old November 22, 2019, 07:57 AM   #8
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@double naught spy- we are talking about enough to notice maybe as much as 5". But, I have never had one take a step after I shoot. So, not shore how relavant it is.
@aquila Blanca- thank you for the thorough explanation. It is a function of geometry and not really parallax related. At least that's not the driving factor. I did switch up rifles this year. Went from a 30-06 Remy 7600 to a Vanguard in 6.5cm. Hopefully I have the chance to determine the difference this makes.
To be clear. Nothing scientific up to this point. As an example. My last kill was approx 35yds downhill about 40 ft facing me. I put crosshair on Adam's apple (if deer even have one) and hit dead center between point of aim and bottom of chest cavity exited belly just past sternum. The result was both lungs and heart gone and no steps taken.
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Old November 22, 2019, 08:59 AM   #9
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FIVE INCHES LOW at close range???

Mechanically, even if you were shooting straight down, your bullet should not impact more than the distance of the scope center bore over your barrel center bore. Most scopes are 1.5-2.5" bore over bore.

That would be a severe parallax issue probably stemming from you mounting your rifle very different for aiming downward than when you zeroed your rifle. That would result in the crosshair appearing to move out of position and you aiming based on this position. I would strongly suggest you take a look at these to learn to adjust your parallax correctly.
https://winchester.com/Blog/2017/10/...cting-parallax
https://www.thetruthaboutguns.com/sc...ax-adjustment/
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Old November 22, 2019, 08:02 PM   #10
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Parallax won't change bullet impact 1/4" at 100 yards. That ain't the problem. From Leupold owners manual.

Quote:
m parallax occurs when your eye is
at the very edge of the exit pupil (Even in this unlikely event, our
4x hunting scope focused for 150 yards has a maximum error of
only 8/10ths of an inch at 500 yards). At short distances, effects
of parallax do not affect accuracy (using the same 4x scope at 100
yards, the maximum error is less than 2/10ths of an inch )
I also zero at 100 yards. The scope is going to be about 1" above the bore so at ranges measured in single digit feet the bullet may impact as much as 1" below the POA. It depends on the exact cartridge and load, but a 100 yard zero will keep the bullet no more than 1" above, or 1" below POA from the muzzle out to 130-150 yards. With most loads around 2" low at 200 yards and 10-12" low at 300 yards. This is pretty much the same with any modern cartridge shooting pointed bullets at 2600 fps-3000 fps. You don't start seeing any real difference until after 300 yards.

I don't like to zero so that the bullets are ever hitting high at any point. That just complicates things at close range and does nothing to help at long range.

Shooting UP or DOWN may cause the bullet to hit a little HIGHER than normal. This is more of a problem with archery than rifles. You'd have to be a LOT higher or lower than the target and the distance would have to be several hundred yards for it to have any real impact.

Good illustrations here explaining what happens shooting up or down. I'm using the top left graphic. If you're elevated 20 yards above the ground it may be 30 yards from where you are located along that leg of the triangle. But in reality the projectile only travels 21 horizontal yards to the animal. That is enough to be an issue with archery gear, not a rifle.

Multiply those numbers by 10 and now you have a situation where it would matter. The range finder may say 300 yards, but in reality it is only 210 yards to the animal. That error would cause you to hit high. It would be the same if reversed and you were shooting uphill. The horizontal distance to the target is closer than it appears.

https://www.google.com/search?q=shoo...w=1242&bih=597

I think you are simply missing low because you're holding the rifle in a non-conventional position shooting at a downward angle.
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Old November 22, 2019, 08:51 PM   #11
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Aiming at a down or up angle makes bullets strike further above the LOS between its two crossing points. The greater that angle, the bullet crosses the LOS the second time further downrange

Shooting with the LOS straight up or down makes the bullet cross the LOS only once at close range.

The angle between the LOS and LOF never changes unless you adjust the sight.

Last edited by Bart B.; November 22, 2019 at 08:58 PM.
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Old November 22, 2019, 09:13 PM   #12
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LOS

I think Doyle's got it, especially if the OP is shooting a large tubed scope with a big bell markedly above the bore. You see that with iron sights as well, the AR being one culprit, the rifle having an "offset" at close range till the bullet reaches the initial LOS.
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Old November 25, 2019, 06:09 PM   #13
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Quote:
When you shoot across level terrain, the bullet starts out basically horizontal, so there's no vertical factor at the muzzle, and the drop is due entirely to the effect of gravity.
While this is an interesting explanation, it is incorrect.

* When a rifle is fired, the projectile leaves the muzzle in line with the bore, then begins to drop immediately due to G.

* When sighting in a rifle, you are trying to intersect two lines, the line of sight (along which you are looking through the sights) and the path of flight of the bullet (initially a line centered on the rifle bore line). These lines form a triangle with the apex at the point of impact/point of aim (assuming rifle is sighted POI/POA)

* When you shoot a rifle, the bullet leaves the muzzle converging with (travelling "upwards" to intersect) the line of sight at your sight-in distance.

* When you shoot with the line of sight level, the bullet leaves the muzzle travelling towards the line of sight. The bore is lower than your scope/line of sight so the bullet has to be aimed "up" in order to reach your point of aim.

* The bullet also drops during flight, so it is actually pointed a little higher than your POA when it leaves the muzzle.

* When shooting downhill at close range like the OP describes, the bullet has not yet crossed the line of sight, so it will hit lower than point of aim by about 1"-1.5".

* Assuming you are holding the rifle in the normal vertical position while shooting downhill, at longer ranges the bullet will strike higher than POA because its line of flight is affected by gravity multiplied by the sine of the angle.
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Old December 25, 2019, 09:19 PM   #14
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I once shot a spruce grouse at about 15 yrds. I placed the crosshair right where the neck attached to the body and touched it off. WOW. Nothing left. I thought at that range that the bullet would take the head off (be high).

Lately I sight in at 3 distances-25,50 and 100 yrds(max on the range), all on the same target.Now I can see where the POI is in comparison to the other ranges. Now if I shot something at less than 100 or whatever I have an idea where it will hit.

I probably will put a little note on the side of the rifle liosting what I found on sighting in.

Bullet usually/always hits higher at 50 and 100 than 25 yards., still rising at 25 yrds.
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Old December 27, 2019, 10:04 AM   #15
Bart B.
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The maximum ordinate (high point) of virtually all bullet trajectories is about 60% of target range.

Use JBM ballistic calculators to estimate your loads trajectories.

https://www.jbmballistics.com/ballis...culators.shtml

Last edited by Bart B.; December 27, 2019 at 10:35 AM.
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Old December 27, 2019, 04:29 PM   #16
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Quote:
I sight all my hunting rifles at 100yds. The hunting I do ranges from 20yds to 150yds+-. Several deer I have taken at close range I have noticed the POI being below the POA. I am shooting from an elevated stand. Is the issue caused by the elevation or by parallax?
Neither.

Shooting at a significant upward angle or a significant downward angle will both make the bullet impact HIGHER on the target than expected, so that's not what's causing the issue.

Parallax errors can result in the bullet missing the impact point, but 5" at close range is a tall order for parallax--I would expect a scope set up for 100 yards to have a maximum parallax error of an inch or less at 20-25 yards. Also, I wouldn't expect it to be a consistent error in magnitude or even direction on the target.

I think what you're seeing is the normal result of the sights being above the bore. At very close range, the bullet will hit low on the target compared to the aiming point because the bullet starts out below the sight. In any rifle, the sights are above the muzzle where the bullet exits. In optically sighted rifles, especially large aperture scopes that are mounted higher than they need to be, the difference between the center of the scope and the muzzle where the bullet exits can be pretty significant. Not 5", but certainly 1.5" and maybe up to something like 3".

The bore is angled upwards to "lob" the bullet to the target, so as the range increases, the bullet initially will be on an upward trajectory as it arcs towards the target. For typical zeroing distances, the bullet will pass the line of sight on the upward part of the trajectory, will peak somewhere about 60% of the way towards the zero, and then will descend back to the line of sight at the zero distance.

Depending on the zero distance and the trajectory of the caliber in question, that initial crossing point during the upward portion of the bullet trajectory could happen a ways down range. At every point before that, the bullet will appear to impact low, and it will be lower on the target the closer the target is to the rifle.
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