Shifting POI

[jusanothajoe]

Recently acquired an HW85 in .22
Got a new Hawke Vantage 4-12X40 mounted this weekend and have been shooting the gun every afternoon this week.
Have heard others talk about first or second cold bore shots might hit low.
I have the opposite problem, my first few shots will be on target, then as I shoot more the POI starts dropping.
Today with the gun cold I shot 5 shots over chrony, avg was 706 fps.
Then I shot 30 more shots. POI dropped 1.5 in at 15 yards.
The shots will group for 3-4 shots then migrate down a little, then repeat.
Immediately after the 30 shots with a 1.5 in drop in POI, I shot 5 more over chrony and avg was 708 fps.
The next day when gun is cold POI will be back on POA.
What say yee ?
I suspect a scope problem BUT, the POI shift is in a straight vertical line ? No horizontal shift at all.
But would a bad scope right itself overnight ?
Did the tissue test on breach seal and could see no movement in tissue.

[Roadworthy]

It sounds like a piston seal to me.  Try your chronograph when it's cold and shooting well.  Check again after it starts dropping.  I suspect your velocity will be down by then.

[jusanothajoe]

It sounds like a piston seal to me.  Try your chronograph when it's cold and shooting well.  Check again after it starts dropping.  I suspect your velocity will be down by then.

If you look in my post, I did try that.
Avg FPS was up a little after the drop ?
But I only did a 5 shot string on both.

[limbshaker]

That's a good one there... Got me! 

Sounds like an overly tight piston seal, or dieseling, but your velocity doesn't back that theory up.

A breech seal breaking in or taking a set can cause stringing, but usually upwards.

Double check and make sure the scope isnt sliding back. Can't hurt to check.

HWs in .22 tend to have really tight chokes often times. May try scrubbing it out real good and lubing your pellets. Usually this will cause some pretty lame accuracy, but what the heck, worth a try.

*Are you shooting off of soft bags? If so you could just be changing your hold (unknowingly) as the bag settles and flattens out. I've caught myself doing it on occasion, and it will shift the POI on some guns.

Try shooting 30 shots from a seated position field target style and see if the problem clears up. Even if the groups arent as tight (I know mine arent!) you should still be able to see if that's the issue.

If all else fails, let somebody else shoot it and see what happens! 

[redcobra04]

Try checking your barrell pivot screws. Had a HW30s doing the exact same thing last week. The FPS was within 1 to 2 deviation. But it kept shooting lower. Checked the screws. Tighted them up and POI back to  normal. Worth a try. Screws just barely moved but made a big difference.

[jusanothajoe]

That's a good one there... Got me! 

Sounds like an overly tight piston seal, or dieseling, but your velocity doesn't back that theory up.

A breech seal breaking in or taking a set can cause stringing, but usually upwards.

Double check and make sure the scope isnt sliding back. Can't hurt to check.

HWs in .22 tend to have really tight chokes often times. May try scrubbing it out real good and lubing your pellets. Usually this will cause some pretty lame accuracy, but what the heck, worth a try.

*Are you shooting off of soft bags? If so you could just be changing your hold (unknowingly) as the bag settles and flattens out. I've caught myself doing it on occasion, and it will shift the POI on some guns.

Try shooting 30 shots from a seated position field target style and see if the problem clears up. Even if the groups arent as tight (I know mine arent!) you should still be able to see if that's the issue.

If all else fails, let somebody else shoot it and see what happens! 

I am shooting off a folded towel on my deck rail.
This gun is a little hold sensitive, but I have repeated this scenario three days in a row.
Always set the stock on the towel in the same place, and have tried "fluffing" up the towel and it didn't make any difference.
One reason I was leaning towards the scope is because once this starts happening, if I try to adjust the scope up it seems like
it takes to many clicks to start correcting it up. But I may be imagining that lol.
Yesterday after POI dropped I adjusted the scope up. Today first 7 shots were 1.5 in high. After 35 shots, back on bulls eye ?
I will try shooting off hand, don't shoot like that much. Thanks

[jusanothajoe]

Try checking your barrell pivot screws. Had a HW30s doing the exact same thing last week. The FPS was within 1 to 2 deviation. But it kept shooting lower. Checked the screws. Tighted them up and POI back to  normal. Worth a try. Screws just barely moved but made a big difference.

Will give this a try. Thanks !

[redcobra04]

Checked every other screw first including the scope mount screws. Thought my scope was messed up also. Have shot a 100 or so pellets since. POI has stayed the exact same. Hope that fixes your problem.

[Kernbigo]

blue thread lock the screws

[dtdtdtdt]

It sounds like a scope / base problem to me as a start.  I don't do break-barrels so can't comment on the barrel hinge suggestions but it seems sensible.  They can be torqued too!

My suggestion it to tear the whole scope base/mounts setup down to the screws, wash all in rubbing alcohol to degrease them.  Get a decent torque wrench - the Wheeler FAT wrench sold at most shooting shops is a good choice.  Reassemble and torque all the screws to recommended levels.  If your rings and bases didn't come with any such information (not uncommon) read the tome below to find what you need. 

Do all that first.  Then, since it is an new scope, I would crank all the knobs end-to-end of their operating range, counting the turns.  Ultimately, recenter the scope and re-zero.  It is possible that the springs and other internals are not quite properly seated after shipping, bouncing around, and possibly sloppy assembly. 

I spent quite a bit of time a couple years ago delving into torques and rifles the Book below is a summary of what I found.  Sorry to do this but I am a bit anal about answering important questions.  This is clearly more than you probably need but I repeat it from an earlier post

I typically use the Wheeler recommendations but here is some more information that you might find useful. 

I use a criss-cross pattern to even out the compression on scopes.  If there is a line of screws, I start at the middle and zig-zag back and forth to each end of the base.  Make sure the rings are well aligned to avoid crimping the scope if you are doing it with the bottom-half of the base installed.  Wheeler actually sells a scope alignment kit with lapping compound to help you get them really straight. I am not quite that particular so haven't gone there (yet!) 

Steel Rings (clamping the scope into rings)  30in-lb,
 
I try to avoid aluminum ones but if that's what's there I use lower torques to avoid stripping the threads in the aluminum:

Aluminum Rings: 25in-lb

Bases - attached to steel action:  40-45in-lb as a maximum.
I sometimes go 5in-lb lower if the screws are slotted rather than Allen or Torx. (to avoid damage when I slip!)   Steel screws through an aluminum block into steel - use steel recommendations. 

Windage (clamping rings to separate bases like Weaver or Picatinny)  45in-lb.

Action Screws - action to stock:  45in-lb.



Below is what I found when I went looking for additional input.

Recommendations from Wheeler FAT wrench manufacturers. They say they have queried scope makers for the following consensus.
 
Steel rings: 35in-lb on steel rings
 Bases: 45in-lb
 Windage (clamping to dovetails or Weaver/Picatinny type bases): 45in-lb.
 Action screws: 45 in-lb.

 Stock / action attachment screws:

 wood or synthetic stocks without bedding pillars: 40in-lb
 wood or synthetic stocks with bedding pillars: 65 in-lb
 Hard service stocks with bedding pillars and military rifles: 65 in-lb.


It also agrees with the straightforward recommendations to use the torque ranges that the screws are designed for. There are two sets of recommendations in the reference books for dry threads and for lubricated threads. Since most gun screws have oil on them, we should use the lubricated thread torque settings. Wheeler specifies the use of lubricated thread torques on the packaging of their current wrench production.

 #6 screw size: 18-20 in-lb - most scope rings
 #8 screw size: 28-30 in-lb - most scope mount bases
 #10 screw size: 40-45 in lb - most scope cross-bolts

 over #10 screw size: 65 or higher in-lb

 
A manual for the Gelmann torque wrench recommended by Anschutz quotes:

 Feinwerkbau: 6-8 Nm or 53-70 in-lb
 Steyr: 5-6 Nm or 44-53 in-lb
 Walther: 7Nm or 62 in-lb
 Anschutz: 5 Nm or 44 in-lb.

 It is not clear what bolts are being tightened here. Given the numbers, I would think that they are for action/stock screws. These values are probably too high for scopes and rings...

 Found this in a Nightforce Scope Owners' Manual

 Nightforce Torque Specifications
• Base attachment screws - 25 inch pounds
• Ring top screws - 25 inch pounds
• Ring crossbolt nut - 68 inch pounds

[dtdtdtdt]

On re-reading my note, it occurred that I was suggesting a whole lot of work before doing the easy one first.  DUH!

Run the turrets on your scope end-to-end as I suggest first.  Since it was straight vertical, elevation might be the culprit  Easy enough to do except for re-zeroing. 

By the way, if you do the torques, I don't find it necessary to use Locktite.  I have an RWS 54 that hasn't moved its scope in years after doing the torque routine. 

You mentioned cold versus hot?  Do you mean storage is cold versus outdoors hot or just the number of shots limbering things up? 

Good luck

[pcp_Dave]

I had something very similar in my Walther Terrus when I first got it.
I would get about 10 shots+- then the poi would move. You could repeat it day after day.
I finally pulled the scope off and starting shooting with the open sights.
No issue since then. That's been almost two years ago.
the scope was a utg 4-12x44 compact. I still have the scope but don't use it.

[Yogi]

+1 Dave hit the nail on the head.

When in doubt blame the scope!
Try if the POI shifts with just open sights.  That way you will know if it is the gun or your scope.
When in doubt blame the scope.

-Y

[nced]

That's a good one there... Got me! 

Sounds like an overly tight piston seal, or dieseling, but your velocity doesn't back that theory up.

A breech seal breaking in or taking a set can cause stringing, but usually upwards.

Double check and make sure the scope isnt sliding back. Can't hurt to check.

HWs in .22 tend to have really tight chokes often times. May try scrubbing it out real good and lubing your pellets. Usually this will cause some pretty lame accuracy, but what the heck, worth a try.

*Are you shooting off of soft bags? If so you could just be changing your hold (unknowingly) as the bag settles and flattens out. I've caught myself doing it on occasion, and it will shift the POI on some guns.

Try shooting 30 shots from a seated position field target style and see if the problem clears up. Even if the groups arent as tight (I know mine arent!) you should still be able to see if that's the issue.

If all else fails, let somebody else shoot it and see what happens! 

I am shooting off a folded towel on my deck rail.
This gun is a little hold sensitive, but I have repeated this scenario three days in a row.
Always set the stock on the towel in the same place, and have tried "fluffing" up the towel and it didn't make any difference.
One reason I was leaning towards the scope is because once this starts happening, if I try to adjust the scope up it seems like
it takes to many clicks to start correcting it up. But I may be imagining that lol.
Yesterday after POI dropped I adjusted the scope up. Today first 7 shots were 1.5 in high. After 35 shots, back on bulls eye ?
I will try shooting off hand, don't shoot like that much. Thanks

A few "just guessing" comments..........
"Always set the stock on the towel in the same place,"
Consistency is a good thing, however I've never been able to get good groups on a "solid rest" with my .177 R9s or HW95 without first putting the open palm of my hand between the stock forearm and the rest.
Try that ans see if it makes a difference.

"if I try to adjust the scope up it seems like it takes to many clicks to start correcting it up."
If your scope erector is too far off optical center after zeroing then the spring tension can be loose allowing the erector to "float around".

Some comments were made concerning stock screw torque, here are the specs from HW..........
Front stock screws = 2.5 Nm is equal approximately to 22.1 in/lbs
Large trigger guard screw = 5 Nm is equal to 44.3 in/lbs
Rear trigger guard screw = 2.5 Nm is equal approximately to 22.1 in/lbs
Notice that these torque specs are INCH pounds, not FOOT pounds

Perhaps your barrel pivot bolt tension is too tight which prevents a consistent latchup every time. Every new HW break barrel springer I've bought had the pivot tension set much too tight.
I set my R9 & HW95 barrel pivot tension so the cocked barrel just Barely FALLS when held horizontal after unlatching the lockup detent.

As a side note........here are the torque specs for the screws of a UTG scope mount (15in/lbs for "vertical screws", 35 in/lbs for "horizontal screws"..........
Uploaded at Snapagogo.com
I've read some torque specs that IMHO (personal opinion) are much too high judging by scope tubes I've damaged using much less torque! Even the BKL mounts I have recommends a max base screw torque of 35 in/lbs but BKL gives no spec for the top strap screws.

Off the wall suggestions..........
1. Excess lube in the receiver that gets pushed in front of the piston seal to combust for the first few shots, however you've debunked that theory with your chrony numbers.
2. Optical center the scope and then see if the poi still wanders up or down.
3. Check the barrel pivot tension and adjust to the cocked barrel "barrel just falls" setting.
4. This is really a perplexing problem for sure!

[mpbby]

“Then I shot 30 more shots. POI dropped 1.5 in at 15 yards.
The shots will group for 3-4 shots then migrate down a little, then repeat.”

I would like to better understand the problem description ..

I didn’t get what the “..repeat” means. Repeat what?

[jusanothajoe]

First, thanks so much for all the comments.
Going to try to answer most of them.

dtdtdt: I own a very good inch pound torque screw driver. I clean and torque every screw.
Scope mount screws torqued to Hawke specs, stock screws torqued to HW specs.
By cold I meant first shots from AC house to outside warming up gun by shooting.
I will try turning the turrents.

Ed: I have been very lucky I guess, all of my guns so far will shoot excellent groups off the towel across my deck rail.
I get much better groups off my towel than off my hand.
I didn't check the scope when I mounted it to see if it was optically centered. But the vertical was only a few clicks off. Horizontal was off by about 6 inches at 15 yards.

Yogi and pcp_Dave: sadly with even 4 eyes I can't shoot open sights anymore, even at 15 yards I can't see the sights AND the target Lol, just too inconsistent. I can change scopes though.

mpbby: Cold the gun will shoot a 3-4 shot touching group, then next shot may drop down 1/4 inch, then 3-4 shot touching group. then next shot drop down 1/4 inch or so, then 2 or 3 or 4 shots touching then repeat the same scenario til POI drops about 1 1/2 inches then it seems to stop dropping.
Again thanks for all the help.

Just checked the turrets.
Turned both turrets all the way one way then counted the "rounds" til they were all the way the other way.
Both had 7.5 rounds of adjustment. both turrets were within a half round of optical center.
I did run them back and fourth the full range a few times.

[mpbby]

Thanks for the explanation.

If this "perplexing problem" (+1) persists..

Although sceptic, the closest I could get to a logical explanation for the - round trip!? - would be relevant temperature changes.

What if you let the equipment outside, literally warming up by itself for some time (half an hour?) – before - start shooting?

[jusanothajoe]

Thanks for the explanation.

If this "perplexing problem" (+1) persists..

Although sceptic, the closest I could get to a logical explanation for the - round trip!? - would be relevant temperature changes.

What if you let the equipment outside, literally warming up by itself for some time (half an hour?) – before - start shooting?

Good idea. will try that also

[T-Higgs]

I suspect something with the lockup or the breach seal. I'm only guessing but it seems logical with the vertical stringing.
Keep us tuned in.

Good luck,
Higgs

[dtdtdtdt]

You've tried everything except changing the scope for another known "good" one.  If things settle down you have a warranty issue. 

I have a RWS scope that my RWS54 finally killed (6+years) that still holds zero and shoots the point of aim but goes in and out of focus on virtually every shot.  A thump on the bell changes it.  I haven't torn into it but suspect a loose lens.  Perhaps the reticle disc is loose in yours?  Maybe a couple light taps around the turret housing might induce the problem??

You mentioned cool versus hot.  Even Alabama probably isn't hot enough, particularly at this time of the year, to shift things much unless you store the rifles in a meat locker.

Another possibility is the viscous properties of the lubricants used.  If they are thixotropic (shear-thinning - the classic example is Ketchup) a few shots could reduce the viscosity and change the piston speed and hence the power.  That doesn't really fly though as you said the velocities remain constant throughout.   

I keep coming back to something in the scope. 

Good Luck!