Range Report POI shift driving me crazy

6mmFan

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Hi guys, I hope you can help. I have had a lot of POI shift problems lately, and it is really starting to p--- me off!

My rifle is a 6mmARTurbo- almost identical ballistically to a 6mmBR- except chambered in an AR-15 type rifle. I was using the Berger 108grain boattail over 28.4 grains of H4895 for a velocity of about 2800fps. This combo has proven to shoot typically under .4” at 100yd (5 shot groups). Previously I had been using the 105 AMAX over 28.8grains H4895. That combo has proven to shoot typically under .5” at 100yd (5 shot groups).

On Saturday I participated in a little friendly “match” that ranged from 100yards to 500yards. I used the 108 grain BT load. The second stage was at 100yd. (pool balls) and this occurred at about 7:30 AM with a temp of about 85 degrees. My zero had been dead on at 100 yd. one week prior at 100 degrees temp. (about 10:00 AM). On the pool ball stage my impact was at the 10:30 position, about 1 inch out (high- left). I made adjustments to compensate at this time. Within 1 or 2 hours the rifle was back to shooting at the original zero. By this time (9:30 AM) the temp was about 97 degrees.

Previously I have done some testing with my 6XC bolt gun to see if the direction of the sunlight is causing POI shift. My findings were that the direction of the sunlight has no effect.

After our friendly “match” I re-zeroed the rifle at 100yards, shooting a .4 MOA group (3 rounds). Then I immediately engaged the 500yd steel. I was missing right on the steel by about .75MOA so I cranked left. Mirage was boiling when I sent the rounds at 500yd. After the correction my group was good, with 4 rounds going into under 3". Then I engaged 100yd paper. Back to missing left by about .6 MOA. I tried again and the shift was exactly repeatable. I cant believe that I am getting that much spin-drift at 500 yards. WTF?? When I was using the 105 AMAX I didn’t seem to have any issues with spin drift out to 500 yards.

What scope am I using? 16X “Super Sniper”. Yes, I know – I know…. (hanging head in shame). I am still waiting for the PSTs to come in.

I used a torque wrench to properly tighten everything. Scope mount is the new Burris AR mount.

Thanks so much for any input / suggestions.
 

JFComfort

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Mar 19, 2009
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Re: POI shift driving me crazy

The only thing I could think of was it may be "light" related but I can't say for sure.

You weren't under the canopy and the gravel was light colored which would make for bright conditions.

I'll have to chew on this one and see if I can brain storm some possible explanations.
 

bohem

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Re: POI shift driving me crazy

H4895 is temp sensitive. Have you tried shooting the load over a chrono at 80* and 100* to see what kind of velocity change you get? It might be just enough to be getting poor groups or throw the harmonics off. Try load testing the same load warm and hot at 100yd and see if the POI shift is repeatable and temp related. I think you might be getting those effects.
 

hexades1

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Feb 12, 2005
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Re: POI shift driving me crazy

Spin Drift is not the cause at these short distances.

Sounds like a canted scope problem to me. Might be the installation, scope itself or the mount.

1. Check that the reticle is in upright position to your rifle. Correct if needed.
2. Check the the scope adjustment knobs are level on your rifle. If reticle is upright and scope is canted you have a scope problem.
3. Try different rings.
4. If 1,2 and 3 do not help then try another scope on your rifle. (if you have one available you could do this first)
5. Try your scope system on another rifle.

Sooner or later you will find out whether the rifle or aiming system is to blame.
 

steve123

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Re: POI shift driving me crazy

I bet it's the same problem I had with mine.The barrel is gradually coming loose from the barrel extension.Send it back to Robert and have him loctite it.
 

6mmFan

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Re: POI shift driving me crazy

I had this barrel tightened down recently so I don't think that is the problem Steve. It is giving me good groups at all temperatures but perhaps that is what is making the POI shift around. I don't think that I had these problems with the other handload so I don't think that it is the scope. I plan to do some more testing to get to the bottom of this.
 

mjb2

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Re: POI shift driving me crazy

Bohem - I thought H4895 was one of Hodgdon's Extereme Powders and was NOT supposed to be temp sensitive ? What makes you say otherwise ?
 

bohem

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Re: POI shift driving me crazy

Hrm, I thought it was temp sensitive and I noticed some changes myself with my 30-06. If it's not changing speed on you then I have another thought.

Can you get the rifle cold, say, 40F and take it out to shoot it quickly? I want to see if the shift happens at cold temps too.

If you clamped the scope down at some temp, zerod at another temp where it's no longer "stress free", now the temp changed and it shoots nice tight groups, but the impact point shifts because the whole package is changing temp and skewing the optics?

I've never seen this happen on a rifle before, but with high grade telescope optics it's not uncommon at all
 

MitchAlsup

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Re: POI shift driving me crazy

A couple of questions:
A) Has this rifle ever shot at long distances before?
B) without displaying the shift effect?

I think the scope is canted, and there is a simple test to see. It involves placing the gun in a vise (gently) verifying that the gun is perfectly verticle with a level. Then use a plumb bob and look at the reticle in the scope and the plumb line at the same time. These two must be identicle. A better test is to then setup the plumb bob in a tree 100 yards away and with the gun still in the vise and still verified verticle, look to see if the reticle remains perfect with the plumb line. Adjust the scope so the reticle is plumb on its verticle axis as needed.

The second thing to try is to verify that the turrets are perfectly verticle and horizontal. There is a shoot the box test where you place a box with 1" targets 4 inches apart over a large rectangular grid. Shoot, dial, shoot, dial until you have shoot at the same PoA and hit every targt on the box. Measurement of the discrepancy between the target and the PoI will tell you if the turrets are correct, if they are accurately machined, and repeatable.
 

steve123

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Re: POI shift driving me crazy

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 6mmFan</div><div class="ubbcode-body">I had this barrel tightened down recently so I don't think that is the problem Steve. It is giving me good groups at all temperatures but perhaps that is what is making the POI shift around. I don't think that I had these problems with the other handload so I don't think that it is the scope. I plan to do some more testing to get to the bottom of this. </div></div>

Are you sure it wasn't the barrel nut that was tightened vs the barrel extension ?

Anyways...I feel your pain man.
 

6mmFan

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Re: POI shift driving me crazy

Thanks for the tips guys. I don't really think that these are the reasons but I am going to investigate what you have said. As I said before I am going to test this load against the last one that didn't have these issues at different temperatures and see what happens.