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Offline tvc15

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odd POI
« on: May 25, 2008, 12:56:17 AM »
   I can understand with different bullets/powders you will get a different vertical POI, but yesterday I shot 20 100 grain and all very tight groups left of center. Switched to 85 grain and all were tight groups , but right of center.

    I'm not talking much, no more than 1.5" off center, I just thought it odd,

    The 100 grain were hornady BTSP and the 85 were sierria HPBT.      TVC15
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Offline Grumulkin

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Re: odd POI
« Reply #1 on: May 25, 2008, 01:36:40 AM »
The bullets were, of course, of significantly different weights.  I've seen similar things happen many times.  I think different torques, recoil and bullet velocities are to blame.
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Offline wncchester

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Re: odd POI
« Reply #2 on: May 25, 2008, 02:35:49 AM »
Odd?  Perhaps, but not unusual.
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Offline mjbgalt

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Re: odd POI
« Reply #3 on: May 25, 2008, 05:07:23 AM »
usually in this situation, i would experience two groups, one a few inches lower than the other, but centered nicely and usually about the same size group.

odd that it went horizontal on you.

but, what the heck! adjust your scope and see if you can't get those little groups toward the center of the target. i mean, it means little in the long run if it's accurate, right?

-Matt
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Offline stimpylu32

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Re: odd POI
« Reply #4 on: May 25, 2008, 05:21:27 AM »
Diffrent bullet / powder combos will produce diffrent harmonics , also your hold on the gun will make a diffrence , with some of my rifles one load will hit as much as 4" from the other even using the same powder but a diffrent weight charge and bullet , so if your only 1 1/2" out from the other call it good for now .

stimpy
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Offline warrior1

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Re: odd POI
« Reply #5 on: May 25, 2008, 05:39:33 AM »
just a wild guess, did you clean your rifle at all between switching bullets?
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Offline tvc15

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Re: odd POI
« Reply #6 on: May 25, 2008, 07:24:24 AM »
This is a hunting rifle, so I'm happy with the set up. I was just curious if this was normal. My limited experience has shown lots of vertical change but no horizontal.  Thanks, TVC15
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Offline LaOtto222

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Re: odd POI
« Reply #7 on: May 27, 2008, 01:20:30 AM »
I am with Stimpy on this. I highly suspect barrel harmonics at play here. Barrels do not stay perfectly straight when a bullet goes down the tube. The barrel actually whips. The whipping will be greater in a thinner barrel, but even a heavy barrel will whip some. Different weight bullets going at different velocities will come out the end of the barrel at different points of whip. What you experienced was the whip was to the extreme left and then to the extreme right with the other load. If you have a hunting weight barrel, you may reduce the effects of harmonics by putting up pressure on the barrel at the end of the stock. Of course if you change the up pressure to the barrel it will effect your point of aim too ( it will shoot higher if up pressure is increased). Many times a rifle will shoot better with up pressure, especially a light weight one, but there is no set fast rule either. That is what makes load development and rifle tuning  interesting. Good Shooting and Good Luck.
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Offline Emmett

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Re: odd POI
« Reply #8 on: May 27, 2008, 07:01:52 AM »
   Good info on the change in barrel harmoics with bullet and powder changes.  Your experience here is nothing new or unusual. Continue to work for tight groups  as usual, watching for pressure as you develop the load. Your aiming point is only that...someting to aim at. You can always change the POI. Pesonally, I don't want my clusters hitting my point of aim while I'm developing a load. It obliterates the exact aiming point I want to concentrate on.
                                                                                                  Emmett
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