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group shifting question

 
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  #1  
Old 01-03-2010, 02:46 PM
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group shifting question

hello,
I have a DE 338 edge with a 5.5-22 x 56 NF. I absolutely love this gun, and it will shoot 3/8 or under all day long. The question I have is:

every once in awhile the group will move 3/4" to the right at 100 yds. still a great group, but 3/4 to the right. i move it over 3 clicks and i am back into the center.

later on i took it out and shot a group between 2 and 2 1/2 inches at 605 yds, near center.

then sometime later i will shoot it and the group is 3/4" left.

I moved it 3 clicks back to the right (where it was at first) and it is now centered again.

I dont believe it to be the gun or scope. i am sure i am doing something to cause this. It never changes during a shooting session. from time to time when i first shoot it the first shot if it is left or right, i correct for it and dont have any issues the whole time i am out. is it cheek weld? is it the way i am holding the gun to the shoulder? is it...?

any thoughts appreciated. thanks

Last edited by lamiglas; 01-03-2010 at 02:53 PM.
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  #2  
Old 01-03-2010, 05:54 PM
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Location: Central Pennsylvania
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Re: group shifting question

Sounds like a scope issue to me. If you can consistently shoot .5MOA, there is nothing wrong from your description in your technique. My guess and it is that at best, is that you have some movement in your turret. It might be a broken tooth or a loose screw or something. Run a box test with the scope and see if there is any deviation in your zero once you are done running around the box.

Tank
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  #3  
Old 01-03-2010, 06:02 PM
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Re: group shifting question

Let us know the results once you've run the scope around the box.
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  #4  
Old 01-03-2010, 09:02 PM
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Location: Alaska
Posts: 3,390
Re: group shifting question

I experience some of this with my rifles also, and I've never really figured it out. Some of these rifles are stable platforms, built and bedded by quality smiths. And I'm using stout one-piece Picatinny bases and stout rings, lapping the lower rings and taking great pains and care to ensure the scope is mounted without strain, and straight with & directly over the bore.

I've read that different lighting conditions can actually cause some differences in point of impact (POI) from point of aim (POA). I've never demonstrated that effect to myself but I've never tried. I believe lighting and air conditions can cause error. Most have experienced the grief cause by heat mirage as the rifle lays still and the reticle moves around the bullseye. I don't know how much error is theoretically possible to due different angles of sunshine or lighting conditions - without heat mirage. It may not be enough to amount to a hoot of difference.

I'm not sure how much your POI is changing versus POA. If it's just a slight correction that varies in different directions on different days, then maybe??? If your POI versus POA moves any substantial distance - like more than 1 MOA, or it continues to drift in any one direction, then I think something else is amiss.

Perhaps some expert on light and the effects it can have on POI versus a fixed POA will sound in and educate on the maximum theoretical error lighting conditions could cause. I'd think the benchrest crowd should know something about this, but maybe not. They do get to take some warmup shots so perhaps they just adjust for the conditions of the day and give it no more thought. I'd sure like to know more about this so I could determine whether or not the amount of shift I experience is cause for concern over the equipment, or beyond my control.

Last edited by phorwath; 01-03-2010 at 09:06 PM.
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  #5  
Old 01-03-2010, 09:17 PM
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Location: IOWA
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Re: group shifting question

From your post, it doesn't sound like it is the rifle.
I am with Phorwath on this one, and think it is light related. Or slight variance in your shooting form.
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  #6  
Old 01-03-2010, 10:28 PM
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Join Date: Mar 2008
Posts: 529
Re: group shifting question

thanks for the replies! I will try a box test next weekend. the reason I have never given that any thought is that i have dialed in for ranges out to 1000 and back and it has alwas held zero just perfectly( elevation wise). i will try the box test and post the results. (wont be until next weekend).

the other wierd thing is that it only changes back and forth this same 3/4" and its never gone beyond that. i have never had this issue before, but i have never had a scope mounted so high. i am using a cheek piece, but wonder now if i am being inconsistent from one day to another. again, thanks the responses and i will post the results.
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  #7  
Old 01-03-2010, 10:45 PM
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Location: The rifle range, or archery range or behind the computer in Alaska
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Re: group shifting question

+2 on the lighting.

Pay attention as to where the sun is when you sight in and then where it is when your group seems to shift.

I have found that when I sight in on extremely overcast days or when the sun is at 12 oclock and another day the sun is at 3 oclock, my POI is right. When it is at 9 oclock, the POI is left. I noticed this first in archery. Every time the sun was left, my groups were left. Every time the sun was centered, so were my groups. When it was right so were my groups. All other seriousd outdoor tournament archers noted the same thing.

I dont know the science behind it but know it is a proven fact that differing lighting conditions affect your perception of target to scope alignment.

Last edited by Michael Eichele; 01-03-2010 at 10:51 PM.
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