Shift in Zero: Diagnosis?

[5RWill]

Well here's the load data for my .308, and i'm unsure why my zero has shifted 1/2 a mil high for the POI.

Hornady 178gr BTHP match
Nosler brass
45gr of varget
fed 210m
COL: 2.865

The optics is a U.S. Optics SN-3, i've had it almost five years now and never had a problem with it shifting zero, tracking, or any function of any sort. That said i have discovered a spec in the top of my scope to the left of the very top of the vertical crosshair which i never noticed. Had never seen it until this year and it can't be seen past 5x. Checked for scratches and saw none. Once adjusted for my shift it still tracked fine to 300. I'll have to do more extensive testing when i get some time.

Here are the groups i shot a year ago with varget, 45gr being left.

Here is what i shot a week or two ago, my friends group is the top left. The only difference that i can think of is powder lot and that these 17 shots fired that day were through a barrel that had recently been cleaned with copper solvent for the first time in a while.

Has me scratching my head.

 

[Bart B.]

No scratch or chip on a lens will cause a spec to appear in the scope's field of view.

If you see a spec at only the lower powers, that means it's only in the light paths going through the lenses at their outer edges, not in the middle to smaller areas used at higher magnifications.

If that spec is well defined and appears sharp (no fuzzy edges), it's on a piece of flat glass at some focal plane inside the scope. Remove the scope from the rifle, hold it at the back end near the eyepiece then bump its front end on the heel of the other hand a few times. Look through the scope and see if it has moved or disappeared. If so, it's some particle that's come off and will continue to come at rest someplace inside the scope.

Does it always appear to be the same size?

A zero shift in your shooting can be caused by a shift in the bedding of the barreled action to the stock. Or bending the scope tube from an accidental bump. Do you loosen your stock screws after each shooting session then retorque them before the next one?

If your friend's groups center a bit different on paper than what yours do, that's normal. No two people hold the same rifle the same way. Each one's zero with a given load will be a bit different from someone elses zero.

 

[Jimro]

Varget is not very consistent from lot to lot. Every time you change lots you need to verify your load velocity over a chrono. Vern Harrison noted that to maintain consistent velocity with Varget he had significant charge weight differences between lots, which is why he transitioned to Re15 for his accuracy loads.

If you didn't switch lots of Varget, then possibly the bore condition came into play, and your zero will shift gradually back to wear it was as the bore fouls.

Jimro

 

[tangolima]

Did you hold the rifle exactly the same?

-TL

 

[Bart B.]

Jimro, everyone gets different muzzle velocity with a given loads' component set and assembly except for the powder lot used. Even if it causes a 100 fps change in muzzle velocity (which is rare, in my opinion), that'll change vertical impact at 100 yards with a .308 Win. caused by bullet drop differences of about 2/10ths inch. Down in the 50 fps range that's typically the most spread in muzzle velocity across rifle powder lots, it's 1/10th inch change in drop at 100 yards will be hard to discriminate.

His target pictures show a horizontal shift in group as I interpret what he posted. Muzzle velocity changes rarely cause a horizontal group shift.

Folks I know who want and get best accuracy were never concerned about all of a given bullet leaving at exactly the same speed. None of them had a problem changing zeros for a given range by a click or two on the sight's elevation knob.

 

[5RWill]

No scratch or chip on a lens will cause a spec to appear in the scope's field of view.

If you see a spec at only the lower powers, that means it's only in the light paths going through the lenses at their outer edges, not in the middle to smaller areas used at higher magnifications.

If that spec is well defined and appears sharp (no fuzzy edges), it's on a piece of flat glass at some focal plane inside the scope. Remove the scope from the rifle, hold it at the back end near the eyepiece then bump its front end on the heel of the other hand a few times. Look through the scope and see if it has moved or disappeared. If so, it's some particle that's come off and will continue to come at rest someplace inside the scope.

Does it always appear to be the same size?

A zero shift in your shooting can be caused by a shift in the bedding of the barreled action to the stock. Or bending the scope tube from an accidental bump. Do you loosen your stock screws after each shooting session then retorque them before the next one?

If your friend's groups center a bit different on paper than what yours do, that's normal. No two people hold the same rifle the same way. Each one's zero with a given load will be a bit different from someone elses zero.

The spec doesn't appear fuzzy to me i don't think. I can't see it past 5x. I just hope it's not the scope. I have contacted USO and can send it in to be looked at. I also have a mark 6 on the way so that could be used for backup.

I haven't taken my barreled action out ever, though i could now that i have a torque wrench. Last it got taken out i imagine was when i sent it to APA to have DBM put in.

Varget is not very consistent from lot to lot. Every time you change lots you need to verify your load velocity over a chrono. Vern Harrison noted that to maintain consistent velocity with Varget he had significant charge weight differences between lots, which is why he transitioned to Re15 for his accuracy loads.

If you didn't switch lots of Varget, then possibly the bore condition came into play, and your zero will shift gradually back to wear it was as the bore fouls.

I'm sure it's different lots because it's my friends reloading components, he shoots year around and has stocks of everything. So when he runs out of varget he just fills his 1lb jug up from one of his 8lb. I've yet to get my setup going but we're making progress this year. I hope it's partly the clean bore as well.

 

[Bart B.]

If that speck stays in the same place in your scope for a given power if you move the scope around while looking through it, the speck's in the scope.

 

[5RWill]

That's what I figured. Do you think I should let USO look at it? What would you do? My friend is telling me if it's not affecting function to leave it.

 

[Unclenick]

They should be able to clean it and inspect it and put fresh seals and fill gas in place. On the off chance there's a scope problem causing the shift, they should find and correct it. Should be a little like getting a refurb, which may extend its life.

Cleaning the bore can affect pressure which can affect barrel time. The bullet leaving the bore at a slightly different barrel time moves it's position in the phase of the muzzle deflection caused by recoil. Very easy to get that kind of difference as a result.

Years ago a friend of mine had POI on several of his .30 cal rifles shift about half an moa low and right simultaneously. It turned out to be due to the fact he'd finally replaced his loose fitting cloth USMC shooting jacket with a tight adjustable buckle leather one with rubber friction pads, including at the shoulder. He just wasn't having his rifle's butts slip on his shoulder as much during recoil as he had been previously.

 

[5RWill]

I see. I talked to USO a couple weeks back they told me to just let them know if I needed to send it in. Think I'm going to send it in just to let them look at it and use my mark 6 in the mean time.


Hopefully though it's just the bore needs to be fouled once more.

 

[Jimro]

Bart B.

Jimro, everyone gets different muzzle velocity with a given loads' component set and assembly except for the powder lot used. Even if it causes a 100 fps change in muzzle velocity (which is rare, in my opinion), that'll change vertical impact at 100 yards with a .308 Win. caused by bullet drop differences of about 2/10ths inch. Down in the 50 fps range that's typically the most spread in muzzle velocity across rifle powder lots, it's 1/10th inch change in drop at 100 yards will be hard to discriminate.

His target pictures show a horizontal shift in group as I interpret what he posted. Muzzle velocity changes rarely cause a horizontal group shift.

Folks I know who want and get best accuracy were never concerned about all of a given bullet leaving at exactly the same speed. None of them had a problem changing zeros for a given range by a click or two on the sight's elevation knob.

At least one person other than Vern Harrison has reported extreme velocity shifts between lots using Varget. 150-200 fps was mentioned by M14Shooter over snipershide.

Looking at the targets I saw a vertical shift, not horizontal. If that is the case, my bad.

If the scope passes a box test, it should be good and not the source of a wandering zero. If it doesn't pass a box test most likely the springs have taken a set and need to be replaced.

Jimro

 

[Bart B.]

Jimro, if Varget has that much spread in fps for a given charge weight, that's quite a bit. A lot more than I ever observed. With 155's in a .308 Win case, a 200 fps difference is 8 MOA at 1000 yards.

If that spread's seen in the same rifle with all other component lots for the load the same, that's absolutely amazing.

A box test of a scope made by shooting groups needs at worst, a 1/8 MOA rifle, ammo and shooter to make a 1/4 MOA boxing error statistically significant. Boxing the scope on an optical collimator placed in the barrel's muzzle is many times better.

 

[5RWill]

This load is running around 100fps faster than my old load. When i was shooting my berger 175gr BT LR with 44gr of varget i was getting 2618fps. With this load i'm getting around 2700.

Also i'm referring to the vertical shift. My first set of groups is a tad right but the vertical is perfect, the second set is a 1/2 Mil high.

 

[Jimro]

Bart B.

You are right about a collimator being a better option, but with as much shift as is happening here, a box test would be a "quick and dirty" way to eliminate the scope as a potential factor. I'm inclined to think that a USO scope is rugged as heck, but even rugged things break sometime.

Blackops_2,

The one inch shift between your friends group and your group is likely from parallax. Did you adjust the scope to be parallax free at the range you were engaging?

Folks shoot scopes all the time with fixed parallax, or improperly adjusted parallax, and get very tight groups as long as they are consistent with their head placement and cheekweld. But it can cause a "wandering group" issue if anything is changed between firing strings.

Not saying that is for sure the issue, as a 100 fps velocity increase may have bumped your load to a different muzzle placement. Could be a combination of both. If the new load continues to perform as is, then odds are your rifle and scope are fine and you just experienced the reason why we are told to do load workups when changed any component, even the same powder between lots.

Jimro

 

[5RWill]

Yes parallax was set at 100. The top left of the second target is my friend, the center is me, just to clarify. Then the first target i shot a year ago is all me.

Think i might run some of our 165 gamekings through it just to see where they impact.

USO is rugged, things happen though. I haven't mistreated the scope even though it's designed to take a beating. The spec makes me curious though. I'm a meticulous person in five years of shooting i would've noticed it i think.

 

[Mike / Tx]

Not that you have the same thing, but that looks almost exactly like what we had happen to the 25-06 the daughter's 25-06 quite a few years ago when it was still in the factory wood.

It was dead on at 200 for years, then one weekend it was an inch high. No biggie figured new lot of powder, bullets, and primers, might be the cause. I hadn't sen this happen in the past though.

Several more months pass and we were getting ready for hunting and checking the sights and getting some trigger time. Pull out the 25, and whatcha know another inch high. Ok now I ma scratching my head and looking hard at the scope. It was nothing fancy a Leupold VX. We hunted with it through the season and after I shot with a friend and it was high again.

This time I pulled it all the way down to the stock, checked everything and could fine nothing but a slight rub mark on the barrel channel. Took an inletting tool and went at it and cleared plenty of wood out to relieve anything that might remotely be touching. Put it all back together and back to the range I went. Everything was happy now and the zero stayed put for all of about 6 months. Then right back up to shooting high. Aout this time Midway had a sale on some Fajen laminate stocks, so I picked one up and slapped it together one weekend.

To date everything has stayed put except the factory stock. When I pulled it I checked it woth a metal straight edge and had about a business card of light between the mag well and the tip of the forend. About three months later I could easily slide a folded card under the gap. For what ever reason that ting started to warp and just kept right on going. I figured that if it kept it up I would find it in a nice rounded curve someday. It was plenty sealed up and water tight, and was always kept in a relatively dry sate with everything else.

Just thought I would throw this over as it, even as remote as it sounds, might be happening with yours. I have personally never seen it before or since but I cannot say it doesn't happen as I did have it happen to mine.

 

[4runnerman]

I had a spec appear on the inside of one of my scopes also. As it turned out,it was a piece of paint inside the scope that had came loose. Did not affect the scope at all other than I could see it inside scope.

 

[5RWill]

I'm not sure it's the stock as i'm running a McMillan A5, not saying it couldn't make contact but i'm not sure how it would alter.

That said i was just checking the free float and couldn't get a thin napkin under the barrel. Think i've found the cause. Checking my action screws...hell the freaking front action screw is finger loose. Going to take it apart tonight and re-torque it down. Knew that Borka torque driver would be of great use.

Is it 65 in/lbs?

 

[Jimro]

Blackops_2,

Action screws coming loose could do it, however I would expect loose action screws to also open up the group not just cause a vertical shift.

But since your buddy shot a higher group than you that stayed in a tight group, I still suspect parallax.

You say you had the parallax set to 100, is that the numeric 100 on adjustment? If so, I wouldn't trust the numbering system, but would test visually for parallax as the numbers are not exactly calibrated to distance.

Jimro

 

[5RWill]

The ergo parallax is numbered to 800 but I've shot using the 100 marker out to 600. But yes I had it on the hundred marker.

The barrel was also impeded as I couldn't get a napkin under it which seems that I lost my free float. I've never had a parallax issue with this optic.

Called my local smith that bedded it and he said 45/50 inch/lbs. going to Re-torque it tonight and try to shoot before I leave.