POI shift

[bellavite1]

I have this Rem 700 MLR in 338 Lapua.
It's the model that came with the AI stock.
Load is Berger 300gr over 81gr of H1000, 2470fps, pretty mild.
Not my most accurate rifle, but then again I am the limiting factor.
What puzzles me, though, is the POI shift that I experience every time I shoot more than 5 rounds in a row: impact gets lower and lower, walking probably 2-3MOA down from zero by the time I get to round 10.
If I let it cool 5-10 minutes it returns to zero.
It is the most I have experienced with my rifles and it gets me wondering whether it indicates any hardware issue (action screws etc...).
All screws are torqued to specs, action screws, base, rings etc.
So guys, is this to be expected on a24" barrel .338 Lapua rifle or is there something I should be double checking?

 

[will67]

Looks like your optic is resting on the pic rail. Might just be the angle of the photo.

 

[straightshooter1]

With a large caliber like this, it sound like your rate of fire is getting that barrel really hot resulting in a POI shift downward as velocity is probably increasing. You might want to do as I've done by putting a temperature strip, like I got off of Amazon (see pic), on the barrel just in front of the receiver. Then you can monitor the heat. You also might monitor your velocity to see if it is indeed increasing as the barrel heats up.

 

[Huskydriver]

I have this Rem 700 MLR in 338 Lapua.
It's the model that came with the AI stock.
Load is Berger 300gr over 81gr of H1000, 2470fps, pretty mild.
Not my most accurate rifle, but then again I am the limiting factor.
What puzzles me, though, is the POI shift that I experience every time I shoot more than 5 rounds in a row: impact gets lower and lower, walking probably 2-3MOA down from zero by the time I get to round 10.
If I let it cool 5-10 minutes it returns to zero.
It is the most I have experienced with my rifles and it gets me wondering whether it indicates any hardware issue (action screws etc...).
All screws are torqued to specs, action screws, base, rings etc.
So guys, is this to be expected on a24" barrel .338 Lapua rifle or is there something I should be double checking?

View attachment 7690569

View attachment 7690570

First off... That scope is garbage...second it looks like it's touching the rail

 

[Newbie2020]

Is it the factory Remington barrel? What contour barrel?

unless it’s a thin walled barrel 5 rds shouldn’t be enough to move poi.

With a large magnum I’m always concerned about flinching. 5 rds is plenty to start physically compensating

 

[bellavite1]

Thank you for your replies.
The scope does have clearance from the rail, no contact, definitely a picture angle issue.
Huskydriver, as far as the scope being garbage, it is your opinion and you are entitled to it, but it was not requested, nor it's relevant to the discussion.
The rifle does return to zero after cool down, so, unless I am missing something, the scope is not a factor.
The question was: is such a POI shift possible to such an extent just because of barrel heating?
It is indeed a large caliber and the barrel is quite hot by shot 10, I just never experienced it to that extent...
It is not a problem, since the rifle as such is not meant for "rapid firing", just a curiosity.
Sraightshooter1, thank you for your suggestion, I might definitely try that.
Actual question, though: I get that velocity might increase because of barrel/chamber heating, but why would it cause a downward shift?
I would expect a higher POI.
If my POI is lower, either my velocity would be lower, or my barrel should be flexing downwards?
I am not dismissing this, God knows there's plenty I need to learn, but please, help me understand the mechanics of it.
Or, maybe, by shot 10 it is just me flinching...

 

[Terry Cross]

With a large caliber like this, it sound like your rate of fire is getting that barrel really hot resulting in a POI shift downward as velocity is probably increasing. You might want to do as I've done by putting a temperature strip, like I got off of Amazon (see pic), on the barrel just in front of the receiver. Then you can monitor the heat. You also might monitor your velocity to see if it is indeed increasing as the barrel heats up.

View attachment 7690949

No.

./

 

[Baron23]

No.

./

Feeling a bit verbose today, Mr Cross

 

[bellavite1]

No.

./

Well, that was helpful...

 

[Terry Cross]

I have this Rem 700 MLR in 338 Lapua.
It's the model that came with the AI stock.
Load is Berger 300gr over 81gr of H1000, 2470fps, pretty mild.
Not my most accurate rifle, but then again I am the limiting factor.
What puzzles me, though, is the POI shift that I experience every time I shoot more than 5 rounds in a row: impact gets lower and lower, walking probably 2-3MOA down from zero by the time I get to round 10.
If I let it cool 5-10 minutes it returns to zero.
It is the most I have experienced with my rifles and it gets me wondering whether it indicates any hardware issue (action screws etc...).
All screws are torqued to specs, action screws, base, rings etc.
So guys, is this to be expected on a24" barrel .338 Lapua rifle or is there something I should be double checking?

I do not think your POI shift would be related to torque values or optic. If either were true, your POIs would not be magically returning to the original zero.

If your stock was an open top forend, I would throw out the possibility of image shift due to heavy heat mirage coming off of your poor rifle after spanking 10rds of Lapua Mag out in one string. However I think your stock would minimize this effect due to the closed top handguard.

So.... My first guess would be you have a factory barrel that harbors a bunch of stress.
When your barrel gets hot, the thermal expansion is very uneven which is causing your POI shift.

Word had it that big Green and most other factory production rifle companies have roughly $50 to $75 in each barrel including the barrels on their "specialty" guns.

Many of these barrels can actually shoot respectably but when heated up, they move A LOT. You have already all heard of this with light contour barrels on hunting rifles, thus dictating letting the barrel cool after 3'ish shots to insure you actually leave with a decent cold barrel zero.

My experience is that going to a heavier contour factory barrel does not magically avoid this.

Note that premium barrel blanks from Krieger, Bartlein, Hawk Hill, etc. will not exhibit POI shifts due to thermal expansion. Their manufacturing method insures minimum stress blanks. I have personally watched even featherweight barrel contours from these makers chambered in 30-06 and .300 Win Mag get smoking hot and not shift at all. Then again, all of these barrel blanks cost north of $300.00 before you even think about chambering, threading, muzzle work, etc. compared to the $50 to $75 factory tube.

If you sort it out, be sure to update your thread with the remedy and details. I'm putting my bet on the above culprit.


./

 

[FisherT&C]

I would switch to a 250 grain and rework up a load. 300 may just be too heavy for that rifle, it tends to be for a lot of .338s.

 

[Terry Cross]

Feeling a bit verbose today, Mr Cross

Had to Google "verbose". Thought it had something to do with veins in my legs or something.

So in the case of my above reply to that gentleman's post, I would say Yes.

When I do come around here and am stupid enough to comment on certain posts, I usually want to go slam my hand in a car door or ask my buddy to kick me in the nuts afterwards.

Most likely I am a victim of inbreeding and innate stupidity thus causing 99% of my own woes but at least I am in good company here.

**Gonna use "verbose" a lot now. Sounds sophisticated.

./

 

[Terry Cross]

I would switch to a 250 grain and rework up a load. 300 may just be too heavy for that rifle, it tends to be for a lot of .338s.

With total respect, please explain how changing his projo weight could possibly remedy his POI shift after 10rd string?

I can see how changing from 300s to 250s might shoot better groups in his rifle but this seems totally disconnected to any POI shift.

./

 

[Baron23]

Had to Google "verbose". Thought it had something to do with veins in my legs or something.

So in the case of my above reply to that gentleman's post, I would say Yes.

When I do come around here and am stupid enough to comment on certain posts, I usually want to go slam my hand in a car door or ask my buddy to kick me in the nuts afterwards.

Most likely I am a victim of inbreeding and innate stupidity thus causing 99% of my own woes but at least I am in good company here.

**Gonna use "verbose" a lot now. Sounds sophisticated.

./

hehehehe.....that's twice I've sent you to the dictionary (can't remember what the other word was).

Don't think of me as being pretentious, just think of me as your one man vocabulary coach! haha

Have a great day, Terry.

 

[Terry Cross]

Well, that was helpful...

Sorry.
He was on to something regarding barrel heat.

Then.....

Then he mentioned the ammo temp being the culprit.
Then he mentioned the vertical displacement downward due to increased velocity.
Then I questioned why I'm still looking at this forum instead of concentrating on shit I need to do.

Gonna get one of those big ass rubber bands and keep on my left hand. Anytime I'm tempted to visit here for anything other than Bear Pit, I am going to pop the piss out of my hand.

./

 

[bellavite1]

Terry Cross, thank you.
Now THAT was helpful!

 

[Terry Cross]

hehehehe.....that's twice I've sent you to the dictionary (can't remember what the other word was).

Don't think of me as being pretentious, just think of me as your one man vocabulary coach! haha

Have a great day, Terry.

No worries!
I consider you my lingual overwatch ninja. So stay sharp!

./

 

[THEIS]

Hi,

@Terry Cross
I will bring you some of the bands I was prescribed by my Doctor to stop my brain from racing to the expo for you.
They also make great slingshot bands.

Sincerely,
Theis

 

[Baron23]

Anytime I'm tempted to visit here for anything other than Bear Pit, I am going to pop the piss out of my hand.

NO!!!!!!

You are actually one of the handful of consistently knowledgeable and adult contributors about actual rifle stuff.

Seriously, I'm not trying to blow smoke at you and I'm not a "groupie" of anybody by any measure....but many (incl me) read your rifle/marksmanship posts as a rock solid, credible source of info. So, please do keep it up.

And yes, get the fuck back to work!! haha ;-)

 

[Newbie2020]

@Terry Cross thanks for diving in

 

[FisherT&C]

With total respect, please explain how changing his projo weight could possibly remedy his POI shift after 10rd string?

I can see how changing from 300s to 250s might shoot better groups in his rifle but this seems totally disconnected to any POI shift.

./

A lighter bullet will have a higher velocity node and I believe the rifle could possibly get into positive compensation at higher velocity. I belive it's worth a shot before throwing the credit card at the problem by replacing the barrel.

 

[Newbie2020]

Velocity nodes or flat spots are a myth and are a statistical anomaly caused by (a) cumulative variances in the reloading process combined with (b) insufficiently small sample size.

 

[PracticalTactical]

Since the POI returns to zero after the barrel cools, the problem can only be caused by stress in the barrel that is responding to thermal changes.

There are a variety of methods that can be used to stress relieve barrels without heating it. There is vibratory stress relief and there is cryogenic freezing. Both might cost as much as a better barrel though, so that is really the final solution.

Since you know it happens, I would recommend that you simply try and anticipate the POI shift, as the problem is not going away without addressing the root cause of stress in the barrel.

FWIW, one of the best custom barrels I ever had was cryo treated twice. The POI was incredibly stable regardless of how many rounds went through the group. The barrel was quite expensive at the time.

 

[Terry Cross]

A lighter bullet will have a higher velocity node and I believe the rifle could possibly get into positive compensation at higher velocity. I belive it's worth a shot before throwing the credit card at the problem by replacing the barrel.

I'm sorry.
I guess I'm being thick headed this morning and just can't track with your surmise.
I am still not seeing how this has anything to do with a POI shift from the beginning of his string to the last shot.

If his current 300gr stuff is shooting decently with POI = POA then shifting when hot, how would changing velocity or bullet weight remedy anything?

I agree with you that he should find a remedy by the least expensive means possible but when OP homie is knocking down 10rds of .338LM every time he gets behind the scope, he is already throwing that credit card around unmercifully.

///
Regarding nodes, etc. I am going to also throw in for others here that it is very easy have ammo with huge ass ES/SD but still shoot sub 1/2 MOA @ 100yds in a squared away rifle.
Everything is so flat inside the first couple hundred yds, that you can have a totally suck ass long range load that looks like money at close range. Absolutely, positively convinced because we have seen it so many times. This could probably be worth a thread to itself so don't want to get off course in this one.


./

 

[Mike Casselton]

Hi,

@Terry Cross
I will bring you some of the bands I was prescribed by my Doctor to stop my brain from racing to the expo for you.
They also make great slingshot bands.

Sincerely,
Theis

@THEIS
Do they make the bands in like a 6-1/2 hat size?
I've got a teenager that could wear one and I can snap it when I need his undivided attention.

Unfortunately, it would probably turn out more like this:

 

[Baron23]

Velocity nodes or flat spots are a myth and are a statistical anomaly caused by (a) cumulative variances in the reloading process combined with (b) insufficiently small sample size.

I think he is referring to an accuracy node...well, perhaps really a precision node where the bullet will be exiting at the peak/valley of the vibration and not a velocity flat spot from diff charge weights (or from diff projectile weights???).

Not that I really understand that either as I can't grasp how a vibration in a barrel will only oscillate in the vertical direction which is what we seem to focus on....seems that the vector sum of vibration oscillations can actually end up in any direction of the clock.

But, as I said....I'm sure I'm just not understanding this at all....well, because I'm ignorant and an old idiot to boot! haha

 

[straightshooter1]

Sraightshooter1, thank you for your suggestion, I might definitely try that.
Actual question, though: I get that velocity might increase because of barrel/chamber heating, but why would it cause a downward shift?
I would expect a higher POI.
If my POI is lower, either my velocity would be lower, or my barrel should be flexing downwards?
I am not dismissing this, God knows there's plenty I need to learn, but please, help me understand the mechanics of it.
Or, maybe, by shot 10 it is just me flinching...

It's been a while since I did a little research regarding this phenomenon, so bear with me for not providing much detail. This issue of POI shift downward with increased velocity has to do with the bullets barrel time and the way the barrel/muzzle moves as the bullet travel down the bore. As the bullet travel down the barrel there's some movement of the muzzle in an upward direction (I don't recall the details why). So, with shorter barrel time, due to increase in velocity, the projectile is going to leave the muzzle earlier in that upward movement resulting in a lower POI. And I'm read somewhere that distance to the target also effects this phenomenon to where one might not see a shift at all in some cases.

No doubt, it's more complex than this as other factors are also at work including, for example: some change, however slight it might be, in harmonics just due to heat.

Bottom like is, you've got a heat issue you need to address with your gun. If you monitor it, then you can better manager your rate of fire and keep from having much, if any, POI shift.

 

[Terry Cross]

It's been a while since I did a little research regarding this phenomenon, so bear with me for not providing much detail. This issue of POI shift downward with increased velocity has to do with the bullets barrel time and the way the barrel/muzzle moves as the bullet travel down the bore. As the bullet travel down the barrel there's some movement of the muzzle in an upward direction (I don't recall the details why). So, with shorter barrel time, due to increase in velocity, the projectile is going to leave the muzzle earlier in that upward movement resulting in a lower POI.

I am quite skeptical of this.
If the system is moving vertically to a different degree due to barrel transit time, then the decreased barrel time (difference between shot #1 and shot #10) would have to be huge percentage wise to change the measured displacement of the system 2 to 3 MOA.

I am going to throw it out there that he is likely not getting any meaningful velocity increase between his 1st and 10th round.
Barrel heat alone will not increase velocity.

Heated ammunition can cause some increase in velocity but most modern propellants are not that temp sensitive anymore. Even with older powders, the OP would need to need to let each round sit in a hot chamber for a few minutes to raise the propellant temperature before firing.

And I'm read somewhere that distance to the target also effects this phenomenon to where one might not see a shift at all in some cases.

Why?
If your research results were correct, the distance should have nothing to do with an angular shift due to barrel time. To isolate and look at your theory, I would take any difference in ballistic curve off the table. The angular shift in departure should be present across all distances.

The OP stated that his POI was shifting 2 to 3 MOA down as the barrel heated. If that was at 100yds, it should be present with a value of 2 to 3 MOA at any distance past that or closer than that.

It also does not bode well that "one might not see a shift at all in some cases". The same tangible reaction should occur every time the same dynamics are played out in the system.

What am I missing in your explanation?


./
ETA: I forgot to include that the OP states he is running a mild load of H1000. That is one of Hogden's Extreme Series powders that are specifically tweaked to minimize temperature related burn rate sensitivity. This would further bolster my opinion regarding velocity gains and barrel time decrease due to any potential heat transferred to the loaded rounds.


.

 

[Huskydriver]

Thank you for your replies.
The scope does have clearance from the rail, no contact, definitely a picture angle issue.
Huskydriver, as far as the scope being garbage, it is your opinion and you are entitled to it, but it was not requested, nor it's relevant to the discussion.
The rifle does return to zero after cool down, so, unless I am missing something, the scope is not a factor.
The question was: is such a POI shift possible to such an extent just because of barrel heating?
It is indeed a large caliber and the barrel is quite hot by shot 10, I just never experienced it to that extent...
It is not a problem, since the rifle as such is not meant for "rapid firing", just a curiosity.
Sraightshooter1, thank you for your suggestion, I might definitely try that.
Actual question, though: I get that velocity might increase because of barrel/chamber heating, but why would it cause a downward shift?
I would expect a higher POI.
If my POI is lower, either my velocity would be lower, or my barrel should be flexing downwards?
I am not dismissing this, God knows there's plenty I need to learn, but please, help me understand the mechanics of it.
Or, maybe, by shot 10 it is just me flinching...

Have you tested another scope?

 

[Terry Cross]

Have you tested another scope?

Never a bad idea because it is so easy to do without changing anything else in the equation.
I don't think it is his optic but would absolutely love to be wrong on this one for his sake.

.

 

[straightshooter1]

I am quite skeptical of this.
If the system is moving vertically to a different degree due to barrel transit time, then the decreased barrel time (difference between shot #1 and shot #10) would have to be huge percentage wise to change the measured displacement of the system 2 to 3 MOA.

I wasn't suggesting that the system moved to a different degree for as far as I know the movement is constant; difference only being when the barrel time where the projectile leaves when the muzzle is at a different point during the movement.

I am going to throw it out there that he is likely not getting any meaningful velocity increase between his 1st and 10th round.
Barrel heat alone will not increase velocity.

Hmmm??? It's simply my understanding that if you get the barrel hot enough, it does. And I don't know just how hot a barrel has to get before one sees any significant difference.

I any case, I thought it'd be a good idea for the OP to check and see if there was any significant difference that might be involved.

Heated ammunition can cause some increase in velocity but most modern propellants are not that temp sensitive anymore. Even with older powders, the OP would need to need to let each round sit in a hot chamber for a few minutes to raise the propellant temperature before firing.

Very true. But the hotter that chamber gets the faster the cartridges heat up. ¯\_(ツ)_/¯

Why?
If your research results were correct, the distance should have nothing to do with an angular shift due to barrel time. To isolate and look at your theory, I would take any difference in ballistic curve off the table. The angular shift in departure should be present across all distances.

The OP stated that his POI was shifting 2 to 3 MOA down as the barrel heated. If that was at 100yds, it should be present with a value of 2 to 3 MOA at any distance past that or closer than that.

It also does not bode well that "one might not see a shift at all in some cases". The same tangible reaction should occur every time the same dynamics are played out in the system.

What am I missing in your explanation?

That was my thinking too and found the explanation counterintuitive. I vaguely recall it had something to do with trajectory and the line of sight. It's been a long time and I'll have to revisit this to see if my very old neurons are working well. . . or not.

ETA: I forgot to include that the OP states he is running a mild load of H1000. That is one of Hogden's Extreme Series powders that are specifically tweaked to minimize temperature related burn rate sensitivity. This would further bolster my opinion regarding velocity gains and barrel time decrease due to any potential heat transferred to the loaded rounds.

Yeah, I was aware. There just wasn't much detail in the OP to have a better understanding what might be going on. . . . other than it sounded to me like definitely some kind of barrel heat issue and the only thing I'm aware of the that might cause a lower POI is that change in barrel timing.

When I first read about this "barrel time" issue with a hot barrel I tried it out on my .308 with a 20" factory sporter barrel and heated it up (not hard to do here in AZ) and sure enough, my POI shifted down. But I didn't have anything close to 2-3 MOA as the OP did.

 

[bellavite1]

Have you tested another scope?

Matter of fact I did, I went as far as sending the scope back to Vortex for testing, but they couldn't find any issue (or at least they said they didn't).
Either way I got them to replace it with a brand new one.
No change...

 

[Mk16.5]

First, I am not an expert at anything, but I did read Frank B. saying that a shot out barrel will start throwing fliers after about 20 rounds after a cleaning. maybe not germaine but It did occur to my addled mind.

 

[jsthntn247]

Stress in the barrel when hot is making it walk. Had some do the same thing. Get it rebarrelled with a premium blank.