Help me diagnose my point of impact shift

3of30

3of30

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So I will start with some back story,

What I shoot: 308 AIAT , 175gr TMK, Varget, Lapua Brass.

4 weeks ago I was at the range just practicing various positions, my rifle and my loads were preforming well and point of impact was exactly where it has been for the past year all the way out to 1200m where I was able to land 2 out of 3 shots on a 12" gong.

Fast forward to the next weekend, there was a PRS match on the Saturday, so I headed to the range Friday night to check my zero before the match on Saturday. As I began shooting at 100m, I noticed my shots were hitting roughly 0.5 Mill low. Shot out to 500m and the 0.5 mil seemed fairly consistent. Slightly confused I chalked it up to atmospheric conditions (even though I knew that was not the case). The first stage of the match confirmed that I was still shooting low and I spent the rest of the match chasing my zero, which is pretty hard while shooting off of a swinging tire... Went home a little disappointed and thought I must have slacked off on a batch of my reloads, maybe not enough powder to explain the drop.

I reload with an old Dillion balance beam scale that I bought used, so who knows what the calibration is like on the thing.

Today I went back out to the range with my chronograph to see what was going on with my loads. (expecting to see low velocities and high SD's)

Here are my results from 4 10 round strings:

Series 1 Shots: 10
Min 2648 Max 2678
Avg 2665 S-D 10.1
ES 30

Series 2 Shots: 10
Min 2644 Max 2664
Avg 2655 S-D 7.1
ES 20

Series 3 Shots: 10
Min 2646 Max 2682
Avg 2663 S-D 11.5
ES 36

Series 4 Shots: 10
Min 2645 Max 2680
Avg 2659 S-D 11.3
ES 35

All in all, I was quite happy with my velocities and SD's (same average velocities I have always seen from my rifle). I was grouping Sub MOA at 100m and 200m. But still all 0.4 Mil low!

My scope is tight, and so is everything else on my rifle. My only thought is that my barrel could use a cleaning, but I am not sure if this could cause a POI shift like I am seeing without a change in velocity?

Any insight would be greatly appreciated. Should I just Re zero my scope and adjust my dope, call it a day?

Or clean my barrel and gather more data?

Thanks

Matt
 
What time of day did you zero and which way does the range face. And did you zero with the same glasses or sunglass that you shot the match with ??
 
Why not just change your scope zero to the current POI? If you are trending low, add more up....

Stuff changes, things move.... go with what the target tells you. As long as it is consistent, change the scope setting.

Something as simple as light can have a profound change on your POI at distance.

Jerry
 
furtaker said:
What time of day did you zero and which way does the range face. And did you zero with the same glasses or sunglass that you shot the match with ??
Click to expand...

I'm not sure what time it was, since I first zeroed this load last year. My zero always stayed the same no matter the time or glasses I was wearing until two weeks ago
 
Mystic Precision said:
Why not just change your scope zero to the current POI? If you are trending low, add more up....

Stuff changes, things move.... go with what the target tells you. As long as it is consistent, change the scope setting.

Something as simple as light can have a profound change on your POI at distance.

Jerry
Click to expand...

Thats what I will end up doing. I just would like to understand how one day I suddenly drop 0.4Mil without explanation
 
What scope/mount are you using????

Pray tell,what scope/mount are you utilizing??
In my limitd experience in such things, the likely culprit is scope or scope mount. Your AI/AT unlikely to change, your ammo looks consistent. You didn't mention the scope/mounts.....Are you certain that isn't the issue?
 
big bear said:
Pray tell,what scope/mount are you utilizing??
In my limitd experience in such things, the likely culprit is scope or scope mount. Your AI/AT unlikely to change, your ammo looks consistent. You didn't mention the scope/mounts.....Are you certain that isn't the issue?
Click to expand...

It is a vortex razor on vortex rings. I will have to check the torque on the screws
 
Ryan.M.Anderson said:
Have you checked the torque on the barrel set screw or remove and add the barrel back again?
Click to expand...

Interesting, my rifle never came with a manual. I will have to look up the spec. I've taken the barrel on and off a few times with no zero shift. But maybe the last time I did (just before it shifted) something is off, possibly the set screw
 
Check that everything is tight as it should be, action screws too, re-zero, if it starts doing it again or even more in similar light conditions, maybe its a scope problem. It can be hard to figure out.
 
3of30 said:
Interesting, my rifle never came with a manual. I will have to look up the spec. I've taken the barrel on and off a few times with no zero shift. But maybe the last time I did (just before it shifted) something is off, possibly the set screw
Click to expand...

The standard amount is 49 inch pounds on the set screw. Some guys down in the States (including Frank Galil) say they are doing 60 in.lbs now as they say it helps with their zero shifts.

This would be mine main culprit.
 
Round count? Ever think of throat erosion from use? Consider re-mic'ing the chamber and chasing the lands? Adding a thou or two of jump could be impacting you.

Just a thought.
 
Key word = refraction.

Old saying is lights up sights up.

My question is what was the cloud cover like?
How did the lighting conditions change between shooting events?
Where was the sun relative to the target in each case.

The bullet always goes away from where the sun is... when the sun is bright shots go far away from sun. When a cloud covers the range, shots will hit toward the sun.

You will usually find about 1 MOA shift between bright sun and heavy dark cloud.

If you shoot on a day when big clouds are rolling over the range, assign a target to each lighting condition and shoot groups that way and you wll see this happen even at 100 yards.... if you have a rifle accurate enough to see it of course. If you have a 1.5 moa rifle, don't bother with this test.
 
3of30 said:
Thats what I will end up doing. I just would like to understand how one day I suddenly drop 0.4Mil without explanation
Click to expand...

If the 0.4 Mil was consistent throughout the ranges you were shooting at, then the problem would have to be something that acted consistently. If the problem was wind/temperature/pressure related, I would have expected that problem to become compounded at longer distances. Mirage or refraction as others have suggested, would have a fairly similar effect (if measuring by angle) at all ranges.

But to be honest 0.4mil is not a huge difference, and well within the range of difference I see from morning to night, hot to cold, summer to winter, etc etc.

When i encounter a consistent variance like that from what I was expecting, I note it in the log, adjust the scope, and forget about it.

If the variance was moving around throughout the day, or performing inconsistently at different ranges, then it would be very different matter.
 
I don't use mils, but if I believe .1mr is about 1/3 moa. If this is the case, .5 would be 1 2/3 minutes.
If this is correct, just adjust the scope and make a note of it in your record book.
I'm surprised that over the past year you haven't noticed any changes in your elevations.
At long range, you can get changes of that magnitude from morning to evening.
 
I don't use mils, but if I believe .1mr is about 1/3 moa. If this is the case, .5 would be 1 2/3 minutes.
If this is correct, just adjust the scope and make a note of it in your record book.
I'm surprised that over the past year you haven't noticed any changes in your elevations.
At long range, you can get changes of that magnitude from morning to evening.
 
tiriaq said:
I don't use mils, but if I believe .1mr is about 1/3 moa. If this is the case, .5 would be 1 2/3 minutes.
If this is correct, just adjust the scope and make a note of it in your record book.
I'm surprised that over the past year you haven't noticed any changes in your elevations.
At long range, you can get changes of that magnitude from morning to evening.
Click to expand...

I do notice elevation changes Due to atmospheric conditions, but these I account for in my pre shot calculations. As of right now it is 0.4 Mil low no matter the conditions.

Thank you all for your input. I think the most likely culprit is the barrel set screw torque as I have now researched. Currently looking for a good torque wrench, then I will check back with the results
 
3of30 said:
I do notice elevation changes Due to atmospheric conditions, but these I account for in my pre shot calculations. As of right now it is 0.4 Mil low no matter the conditions.

Thank you all for your input. I think the most likely culprit is the barrel set screw torque as I have now researched. Currently looking for a good torque wrench, then I will check back with the results
Click to expand...

I would recommend a Borka tool kit - I am very happy with mine. A lot of guys also use that Stanley thing (Fatmaxx maybe?).
 
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