POI shift - what happened here

[chawk]

I was out shooting today with a rifle that has always been extremely consistent. The first round of the day and the following 4 rounds were results as anticipated. The rifle was shooting as it always had, with good accuracy and POI was what I have come to expect from this rifle/load combination. One shot later the POI is 0.5 mil high. Thinking to myself I'm not sure what I did there, but I chamber another round and send it. High again. 5 more rounds all high. I'm really scratching my head at this point so I switch targets and put a group on paper at 300 yards. The group is 1/2 MOA but 0.5 mil high.

The rifle is a 300WM with 220 rounds down the tube. manners stock, bighorn action, Krieger barrel, seekins rings, nightforce scope.

The rounds were all loaded in the same batch. No components were changed and all were from the same lot. Norma brass, fed 215 primer, h1000, Berger hybrid 215.

I ran out of daylight and couldn't really dig into it any further. I checked the rifle when I got home and everything seems good. I'm planning to shoot it at 100yards to re-zero and then shoot it at 800 yards which is the farthest I have access to. I'm curious if it will need as much elevation as it always had to get to 800.

Any thoughts or suggestions about what could be going on or what I might want to check would be appreciated.

Thanks

 

[bigngreen]

I would run the scope a few revs up and down and see if your zero resets, I've seen Nightforces move zero on a few occasions now.

 

[chawk]

Bigngreen

Thanks for the suggestion. When I get back to the range I'll shoot the rifle prior to doing this. If the zero is off I'll run the turrets up and down some and see if that moves POI.

 

[WildRose]

Is there any chance you changed powder lots or primers loading this ammo?

I'm with BG though, the first thing that came to mind is that one way or another your scope moved so I'd double check the torque on the mounting nuts.

If this repeats and you find no other culprit I'd pull the action out and see if perhaps some debris has gotten under the barrel. It wouldn't take much pressure to cause a significant shift as the barrel heats up and expands.

 

[chawk]

WildRose,

Thanks for the reply.

All rounds were loaded in the same session and all brass, primers, powder and bullets were from the same lots. That's one of the few things I'm certain about.

I'll triple check all the mounting nuts. I checked the action bolts, but the idea that something might have gotten under the barrel is one I hadn't considered. I'll look into that.

 

[Greyfox]

Before taking the rifle/scope apart, or changing any parameters, I'd verify the change on a different day. Not to say something may be going on with the mechanics of your rifle, but it's always good to recheck just to make sure there isn't something going on with your form. I'd pay particular attention to cheek weld pressure which is a common cause of elevation POI changes if a subconscious change in pressure is applied to the stock.

 

[gohring3006]

One thing I noticed in my form.
Is I did a load work up and rifle zero in the summertime, and the other day I was shooting in 15 degree weather with a heavy coat, and my poi shifted high about 2 Moa.
I took the coat off and it went back to its original zero.

 

[Bravo 4]

As Greyfox said, confirm it on another day. And bigngreen is right as well that even a Nightforce can go bad, it's just not as likely as most other brands.
Something I didn't read about is any time between groups to cool down...?
If it shoots poi as it should on your next outing I would suspect a heated barrel may be changing the poi. Example: While shooting in a long range match in Missoula MT a hand full of years back with an issued M24, I realized that somewhere at rounds 10-12 the poi would drop 1 solid moa. I would run strings fast and it was predictable, but only enough that at round 10 I knew to look for it and knew it would drop that one round and I would have to add 1 "click" up at get it back into the 10 ring.
Point is, if the zero is back to normal next outing either heat it up and see or just don't let it heat up again. If it's a hunting/target rifle and not for competition then I wouldn't let it heat up again anyways. Hunting rifles I won't usually shoot more than 3 rounds at a time before it cools. I don't care what it'll do at round 5 or 10 or 20, totally different with a rifle that'll get 20 round strings through it.

 

[86alaskan]

you may be experiencing what a lot of target shooters get with their rigs. seems through my reading that most rifles will see a speed increase somewhere around 200ish rounds, that could be a 100fps+ faster. It makes sense to me that that could very well be what happened. so, after doing a thorough checkout to rule out other problems, I'd put it through the chrono and see.

 

[Canadian Bushman]

Same exact thing happen to me just last weekend with my 6br. Me and a buddy were trying to shoot a jar of tannerite in the 600's and after about 10 shots my impacts just jumped a minute left. I couldnt believe i missed a wind shift that big so i waited for it to die and the next three followed.

A day later went out with my dad and a minute left.

Two days ago went out again 1 minute left.

All groups were sub 1/2 moa or just over (wind), and my last outing i had three groups with two different lots of ammo printing just over and inch in the low 600's.

I really dont know what caused it, i just re-zeroed the scope and will be paying close attention to the wind on the next few outings.

Scope is a NF 5.5-22x55 nxs in 3 rings on a bedded NF steel rail.