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Miseryguts

Shift of POI with position

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Hi, I have a semi auto 22 with a mid range scope(with parallax/focus adjustment) zeroed at 75 yards on my own range off my portable table and chair. This gives a POI 1 inch high at 50 yards as predicted by my ballistic program. On a visit to the range of one of the clubs I belong to, shooting at 55yards (their 50m, and longest) off their bench and seat which I find very difficult to get a good sight picture from, POI was 2inches high! Is this down to the awkwardness of the shooting position? Has any one else had this sort of change?

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This sort of thing is often,though not always accurately,refered to as a 'parallax' issue....your zero is 75y,but what is your adjustable parallax set at? If it were 75y and you shot at 55y,there might be a little parallax related error.

 

There should be no substantial POI /'parallax' issue IF you look directly/straight down the centre of the scope tube (axis).BUT:

 

The problem arises if the eye position changes wrt this central axis.....and that is ever likely if your shooting position changes and becomes 'difficult',as you describe-you are unlikely to be looking directly down the scope axis,as you were zeroing at 75 in a comfortable position.

 

gbal

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Had a look at my own 22lr ballistics and altered to a 75 yard zero,,,,this then shows 1.75 high at 50,,,,1.6 at 55,,,,,,,so getting near your 2 inches,,,? In addition 22lr group sizes are generally getting pretty ragged at 75 so have you truly arrived at an accurate 75 yard zero? being honest,,,,,sometime hard to keep match ammo within an inch,,,,,,,,regds,,,,,O

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POI 1 inch high at 50 yards as predicted by my ballistic program.

If you haven't verified that POI in actual shooting on your own setup I don't think you can say you have a POI shift issue.

 

Strelok says my setup would be 1.55" high at 50m with a 75yd zero. As OneHole says the confidence in your 75yd zero being <0.5" out can be difficult to achieve, the range has to be precise, and the statistical confidence interval in the mean POI can be surprisingly large.

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Ok, I admit I may not have been precise enough! According to my laser distance measure, the target that I zeroed on was 77 yards. Attached is the group shot after finalising zero - I think it was 6 shots. At my range I have gongs set out at 50,75(or 77 if you must!)100, 150, and 200 yards. After zero, I checked the POI at 50, and 100 yards, and they agreed with my ballistic program (Shooter). The change in apparent POI was a week later at my club range, which is 55 yards (according to laser)

Ammo is RWS Special match 40gn at a measured MV of 1030fps. A pal of mine hit the 200yard gong(3inch) 4 times out of 5 on the same day as I checked the POI at 50 and 100 - sickening Eh?

 

post-15030-0-11830100-1491925493_thumb.jpg

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My Daughter had a similar thing happen last weekend.

 

.223 at 100 yards - zero was perfect, about half an hour later drew lots for a firing point. Ended up 5 lanes over and on a left hand bench (she's right handed).

 

Awkward to get settled, shot a 5 shot sub inch group about an inch right of zero. Very frustrating, couldn't explain it. Cost her the comp by 2 points.

 

Put it down to a different, uncomfortable position.

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My Daughter had a similar thing happen last weekend.

 

.223 at 100 yards - zero was perfect, about half an hour later drew lots for a firing point. Ended up 5 lanes over and on a left hand bench (she's right handed).

 

Awkward to get settled, shot a 5 shot sub inch group about an inch right of zero. Very frustrating, couldn't explain it. Cost her the comp by 2 points.

 

Put it down to a different, uncomfortable position.

Hi, think your probably right - am hopefully going to to my range today, and will try again with my usual set up and check the POI at various ranges

 

 

Velocitor or RWS match,,,??confusing me now??,,,,,O

 

Hi, this medium does not lend itself to adequate explanation, especially for old gimmers like me. Rifle was zeroed with RWS, the group with velocitor ammo was shot by dialling the relevant elevation. Subsequent sessions on my range/club range which showed the anomaly(to me anyway) were all shot with RWS - I posted the target pic as it was the only one I had to show that the rifle was on zero at the range I indicated, a nominal 75 yards measured with laser at 77 yards. Hope that explains things.

 

Thanks to all for taking the trouble to reply

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You certainly can get a change in POI by changing position, I'm always surprised how far off the sights can apparently be for another person shooting the same rifle. A 1MoA change from the expected impact is certainly possible, due to a number of factors, parallax, different supports interacting with recoil etc.

All you can do is shoot again and see if it is repeatable, luckily shooting is fun! :-)

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You certainly can get a change in POI by changing position, I'm always surprised how far off the sights can apparently be for another person shooting the same rifle. A 1MoA change from the expected impact is certainly possible, due to a number of factors, parallax, different supports interacting with recoil etc.

All you can do is shoot again and see if it is repeatable, luckily shooting is fun! :-)

 

Went to my range yesterday, forgot to take the bipod, so had to improvise. This caused a 1moa shift in elevation, and 0.5moa in the windage of the zero at 75 yards.

Rezeroed and shot targets at 50 yards and 100 yards(measured as 102 yards with the Laser)

50 yards shots were 1 to 1.5 inches high - ballistic program predicted 1.5in

100 yards 4 to 4.5 inches low - ballistic program predicted 3.9 in

Ballistic program was with a MV of 1030fps, which was the mean MV the last time I chronographed this ammo

I am sure the variations are down to my crap shooting anyway

Thanks again for all the input from those who replied

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