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Changing POI

 
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  #22  
Old 05-21-2013, 12:18 PM
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Join Date: May 2007
Posts: 268
Re: Changing POI

Hmmm... this reminds me of an issue I had with a custom built 50BMG rifle.
Good accuracy yet POI shifted around.

$2000.00 in components later I figured out that the gunsmith effed up the threads on the barrel...I could actually unscrew the barrel by hand!

Flip the receiver upside down and try your test again. I can only think of one place you can get play from...That is exactly how I resolved my BMG issue....If I am right you are getting a bigger reading off the scope because it is farther from the threads and shows the movement more.

Might be time to take your rifle in to a 'smith.
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  #23  
Old 05-21-2013, 12:58 PM
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Re: Changing POI

So you are saying it could be loose or incorrect threading on the barrel not allowing it to mate properly with the receiver and this can cause the movement when I put pressure on it. Hmmm something to check out.

What exactly do you mean when you said flip the receiver upside down and try it again.
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  #24  
Old 05-21-2013, 03:09 PM
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Re: Changing POI

Quote:
Originally Posted by lisagrantb View Post
So you are saying it could be loose or incorrect threading on the barrel not allowing it to mate properly with the receiver and this can cause the movement when I put pressure on it. Hmmm something to check out.

What exactly do you mean when you said flip the receiver upside down and try it again.
See if you can get the receiver to move the opposite way or get it to move both ways to see if you can double your reading.
maybe clamp the barrel at the shank near the action and try to measure any action movement both ways.
I'm no gunsmith and am just guessing here.
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  #25  
Old 05-23-2013, 08:43 AM
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Re: Changing POI

Ok, after much testing here is what I have found out.

The equipment used for the test was a Remington 700 bluprented and Brux barrel with Nightforce rail the second gun is a Defiance action with a Lilja barrel and Seekins rail, 3 scopes, 2 Nightforce scopes one with Seekins rings the other with Nightforce rings and 1 Lupy MK4 with Badger rings. I removed the stcks from both guns and checked and torqued everything. I then took the 700 and clamped it in a barrel vise and mounted and torqued the first scope on it. I put the crosshairs on a 100 yard target and applied upward and downward pressure on the objective bell checking the movement after each time ( actually the sunshade so that gave a little more leverage) not a lot of pressure but firm. I watched as the scope would move its POA about .4" up and down. I repeated this with all 3 scopes on the first rifle and the results were all the same, so i'm thinging GUN right. Then I switched guns out and tested all 3 scopes on the Defiance and guess what, same POA change with all 3 scopes on the second gun. Now I'm saying SOB whats up now. I then took all 3 scopes (one at a time) with the rings mounted and clamped the rings in the vise of a milling machine, only the scope and rings nothing else. this setup is as stable as I can get. I pushed, pulled, twisted and yanked on the rings and turret trying to see any movement. The setup was rock solid I couldn't see any movement of the crosshair on the 100 yard target. I then took my hand and applied pressure to the objective bell and guess what, the crosshair would move up and down depending on the direction I pushed the bell (I would expect that because I'm flexing the tube) but they wouldn't go back to zero after I released the pressure, there was a .4" **** in POA the same that I was seeing when the scope was mounted on the gun. I repeated this with all 3 scopes and the results were just about the same for each scope. I think the tube is flexing and moving the internals and they aren't returning back to the same point, i'm sure the tube returns back. I dont think something is broken with all 3 scopes I think its a design limitation and just something to be aware of. Any thoughts on this, it would be nice if someone could try the same thing and confirm or disprove what I found.
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  #26  
Old 05-23-2013, 08:58 AM
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Re: Changing POI

Quote:
Originally Posted by X-man View Post
Put another scope on the rifle.
I agree, but during the process I would remove all the mounts looking for even the slightest movement. It could easily be the mounting system, (bases or rings)

What are they?

Jeff

EDIT; we were posting at the same time. What you are seeing I find as normal. I have preached about spending the money for good one piece rails so recoil is not transferred into a scope tube. A good rail epoxy bedded to a receiver will strengthen the action and allow things to stay true. I say use a good one piece rail, bed it to the receiver using a straight edge to assure it is true, and mount it in rings of high quality that are on plane to each other. Then torque it all down as a whole. It does make a difference, but only a difference that few will ever realize.

Jeff
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  #27  
Old 05-23-2013, 09:17 AM
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Re: Changing POI

I did change the scope, 3 scopes on 2 different guns all combinations. The rings were mounted on the scopes and I clamped the rings in a vise that is mounted to a milling machine I don't think you can get any more solid than that and the problem was still there with all 3 scopes. The rings are Nightforce, Seekins and Badger I wouldn't think that all 3 rings would allow the movement. I really think it's the scope internals and the way there designed that cause the error, just something to be aware of that could affect your POI.
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  #28  
Old 05-23-2013, 09:20 AM
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Re: Changing POI

Quote:
Originally Posted by lisagrantb View Post
I did change the scope, 3 scopes on 2 different guns all combinations. The rings were mounted on the scopes and I clamped the rings in a vise that is mounted to a milling machine I don't think you can get any more solid than that and the problem was still there. The rings are Nightforce, Seekins and Badger I wouldn't think that all 3 rings would allow the movement. I really think it's the scope internals and the way there designed that cause the error, just something to be aware of that could affect your POI.
I am agreeing with you. And offering why a stable mount is important to reduce this all we can. I also like the rings as far apart as possible. Also why I never use my hand on top of a scope during a shot to steady the rifle.

Jeff
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