Suggestions on diagnosing zero shift?

[DJL2]

I hit the range this weekend and saw a .6 mil shift in my zero that I cannot explain. Nothing changed on or with the rifle since last fired (and zero verified). I assume I'm seeing an issue with the mount or the optic, but I'm not exactly sure how to isolate/diagnose.

Background:
Rifle: KRG SOTIC
Optic: SIG Tango6 4-24
Mount: Spuhr ISMS

Rifle lives in the safe, muzzle up. It travels to and from the range in the soft case/drag bag supplied with the rifle. I don't abuse it, but I don't baby it either...I just treat it with respect. It'd probably been a good month since I shot it because I've been focused on project rifles. I fired the ammo the system was previously zero'd to and two others. Three five shot groups initially and they all hit consistently .6 mil low. I dialed .6 mil, right on, no issues. The dial was "zero'd" on the optic. I certainly don't recall fiddling with or resetting the dial or zero stop (why would I?).

 

[_Windrider_]

Parallax?

 

[Greg Langelius *]

How you address a rifle and how you manage recoil will each have a consequence regarding POI.

Possibly, your sojourn getting the project rifles up and running could have affected these factors.

Also, I had a scope objective bell get bumped from the top, and it bent the scope tube very slightly, but that did (and should) change the POI upward. It had an effect similar to changing the base slope. Forcibly lifting the rifle by the objective (might) end could do this, particularly if the muzzle hit something on the way up. Grasping at straws here...

Greg

 

[jcmullis2]

Probably a problem with the scope

 

[Dthomas3523]

If you can find anyone locally with a “scope checker”......basically a scope with the internals frozen that’s on a plate that attaches to your pic rail and has a pic rail to mount your optic on it.

You basically mount the plate/scope and your scope. Then line your crosshairs up to the same as the frozen optic. Fire the rifle or otherwise beat on it.

Then make sure they both still line up. If your optic doesn’t, it has a wandering zero.

They are harder to find, but if you have some serious F class or benchrest shooters in the area, there’s probably one floating around.

 

[DJL2]

Thanks for the replies. Definitely some things here I didn't consider and will evaluation/look for/at.

 

[Overclok79]

To hijack this thread, from range day to range day, how much zero shift do you guys normally see? I would assume weather can effect your zero?

 

[Dthomas3523]

To hijack this thread, from range day to range day, how much zero shift do you guys normally see? I would assume weather can effect your zero?

Shooter inconsistency and lighting are the two biggest culprits. I wouldn’t be alarmed with anything .2 or less.

Just check out all the groups people post. One day on the right side of a 1” dot and the next it’s on the left. Or upper vs lower.

 

[Diablo]

My last encounter with a nagging zero shift was due to some Burrus rings used for mounting an ST-10 on my old LE HS Precision HTR that I traveled with, used often, varying weather conditions. Replaced the (Burrus) rings with some steel (Badger) rings and it solved the problem.

Suggest:
--the first suspect is whoever was with you at the range that day, and gave the knob a little adjustment just to screw with your head and see if you noticed.

After analyzing that possibility, then you could
--check action screws for correct length and torque
--check scope mounting and assure it is solid
--check shooter technique, position behind rifle, bipod loaded, etc., etc.
--check scope

 

[DownhillFromHere]

Check your brake. Mine came very slightly loose during a range session and I got a couple tenths shift. A shift of .6 is a lot, though...

 

[Overclok79]

Shooter inconsistency and lighting are the two biggest culprits. I wouldn’t be alarmed with anything .2 or less.

Just check out all the groups people post. One day on the right side of a 1” dot and the next it’s on the left. Or upper vs lower.

How does lightning effect your zero? Can mirage change zero? I shot a match last weekend in crazy mirage and felt like I qas missing when I shouldn't have been.

 

[Dthomas3523]

How does lightning effect your zero? Can mirage change zero? I shot a match last weekend in crazy mirage and felt like I qas missing when I shouldn't have been.

Both mirage and lighting can change zero. For mirage, especially if you zero from prone on a warm day, you’ll have a lot of mirage built into your zero. On those days it helps to get up on a bench and stand behind it.

Here is a video that illustrates lighting changing what we see down range. This can happen when we zero in certain lighting and shoot in totally different lighting.

 

[Dthomas3523]

I should clarify. Lighting and mirage change your perception.

The bullet does what you tell it to do. But if your perception of the target was different either when you zero’d or when you’re shooting, you’ll be telling the bullet to do something you don’t intend.

 

[DownhillFromHere]

How does lightning effect your zero? Can mirage change zero? I shot a match last weekend in crazy mirage and felt like I qas missing when I shouldn't have been.

No, mirage doesn't change your zero - how could it? But mirage can, and does, make the target appear to be someplace other than where it actually is.

In a long and interesting conversation with a representative of one of the major optics companies, we talked about effects of mirage. He described how a colleague mounted a camera on a reticle-equipped spotting scope and centered an 800-yard target in the reticle. The camera was set to do a time lapse sequence, starting early morning and ending later in the afternoon.

Playing back the time lapse showed the target dancing all over, up, down, left, right, in the reticle. The target was not moving. The camera was not moving. But the air in between, acting as a lens, bubbled and boiled and "moved" the target.

It's very obvious to me how mirage affects how we see and address long-range targets. It's less obvious how lighting - sunny vs cloudy, morning vs afternoon, etc. - affects POA vs POI, but objectively I know it does. Because I see the effects on paper, even at short 100-200 yard distances.

Ya gotta be a little weird to wanna get deep in the precision rifle weeds.

Edit: And @Dthomas3523 puts up the video - or at least a representative video. Cool.

 

[seanh]

Think of lighting conditions like you do Parallax in a scope. When Parallax adjustment is poor and you move your head around the scope you see the reticle move. When you change the direction, intensity, color of the incoming light the same effect occurs with relation to the perception of target location.

People forget the atmosphere is a big, very low density lenses. When the direction, intensity, color of light changes so does the perceived location.

I saw this regularly when I worked at an archery range. Had a couple lanes that didn't have lights above the shooter. If you shot well enough you could see the change in zero between those lanes and the ones with good lighting. Had to educate quite a few archers on basic physics.

I'd like to see a test of two identical scopes with the only exception being a sunshade. I would assume that the effect have perceived shift would be less with a sun shade.

 

[Bakwa]

Sig optic. There's your problem.
JK
But seriously, after ruling out the user, environmental factors, rings and base remaining tight, then the scope is the obvious candidate. I'd check in that order though.
Cheers

 

[Overclok79]

Both mirage and lighting can change zero. For mirage, especially if you zero from prone on a warm day, you’ll have a lot of mirage built into your zero. On those days it helps to get up on a bench and stand behind it.

Here is a video that illustrates lighting changing what we see down range. This can happen when we zero in certain lighting and shoot in totally different lighting.

Wow! Thats an insane amount of movement. Is there anyway to be able to predict this? I feel like the only way to is verify your dope in a ton of different atmospheric conditions

 

[Dthomas3523]

Wow! Thats an insane amount of movement. Is there anyway to be able to predict this? I feel like the only way to is verify your dope in a ton of different atmospheric conditions

Not much way to predict lighting and mirage changes.

Sometimes the target is large enough to absorb the error. Sometimes you get sighters. And other times you end up losing a point to get a call on follow up shots.

With mirage, you can dial magnification back until it’s minimal. Or other times you favor low a tenth or three.

 

[Dthomas3523]

Wow! Thats an insane amount of movement. Is there anyway to be able to predict this? I feel like the only way to is verify your dope in a ton of different atmospheric conditions

Also, keep in mind that is a time lapse video. So the change over the day is going to be extreme compared to the change you’ll see while on rifle.

Typically with light refraction issues, it’s when you zero in one extreme lighting and shoot in the other. In which case you can recognize it and adjust your dope accordingly for the day.

 

[HousePlant]

My zero is rarely exactly 100% the same between range visits. Different temp, different wind, different amount of coffee ingested...

 

[PinesAndProjectiles]

I want to add something to the ambient lighting portion of this discussion.

Friday afternoon, I was shooting to the West at about 4pm, when the sun was high in the sky. I was laying on my deck and it was hotter than two rabbits fucking in a wool sock.

I'm shooting right handed, and my left eye would get sore from the ambient light.

I used a towel and covered my head and the eye piece of the scope to block out the sun.

The groups that I shot under the towel, were all about 1/10 of a MIL low. This was consistent, and repeatable.

 

[DJL2]

Check your brake. Mine came very slightly loose during a range session and I got a couple tenths shift. A shift of .6 is a lot, though...

That might have been the total shift going from brake off to brake on, or thereabouts. It’s been tough to get to the range, but I’m curious to see what happens.

 

[COprecisionhunter]

That might have been the total shift going from brake off to brake on, or thereabouts. It’s been tough to get to the range, but I’m curious to see what happens.

I had a self timing brake come loose and had to re-zero after re-install.

 

[Deleted member 113831]

I've seen shifts of more than 2moa (0.6mil) in extreme instances. One was a class at Sac Valley where the entire class saw a 2moa shift to the right, on the 1K range from initial zero in the morning to the last hour of the class when the sun was low on the horizon and shining bright at 90 degrees to the line.

The others were classes at a private ranch where a 800 yard target was shot in the morning down into a well lit valley 7 degree angle. Coming back around to the very same target, the valley is now in shadow and the sun low on the horizon and shining in your face. The entire class was 0.8-1mil high.

Also, RO'd a match in Kettle Falls. Two of the targets (3/4 IPSC) were at the same distance (525 yards). One of them was in an open field, and the other was about 50 yards into the trees and in shadow most of the time.

Shooters consistently shot over the target in shadow. However, at one point the sun was at just the right angle to shine down on the target in shadow, then shooters never saw a difference.

 

[isofahunter]

^^^
Light is Voodoo a wise man once said. I agree with him.