How sensitive to temperature and barometer changes is the POI?

[JD]

I just finished a shooting session and barometer, temp, etc were pretty much the same as the session in question only there was no wind today. I shot in a T shirt, no sweatshirt. Still wretched groups low and right.

I put thin tape on the balance point of my rifle and thin tape on the bag to try the balance point idea again. Didn't help so I went back to putting the front screw at the front edge of the bag.

I decided to forget about figuring it out and worked with scope clicks. Originally I was 16 clicks down and 5 clicks left from optical center. Fine results there, then it all fell apart hence my thread.

So I removed 12 down clicks and added 5 more left clicks and that allowed me to punch out most of a nickle sized green dot on my target. (30 yds)

And I quit while I was ahead. Hope everything is still good when I have a chance to shoot again.

Maybe the scope has problems, IDK. This is my second one from Hawke. First one acted similar. This one is the Airmax which is what they recommended for my RWS 34 as an upgrade from my first Hawke Vantage. I don't have another scope I'm willing to risk, all my other scopes are PB scopes.

[Horatio]

How many pellets the scope have since mounted?

The gun might not shoot first shot cold to same POI?

[Geronimo]

Ask anyone that was at the pyramyd air cup this year if temperature messed with us in any way. We all made adjustments at the sight in that morning in unseasonably cold temps. The temp rose quickly and when the comp started we were all shooting to a different poi than we had adjusted to. The next morning most of us left our scopes alone and did much better. I see a lot of guys keeping their scopes covered to keep them from heating up and it was explained to me that temp makes big differences inside your scope and how the pieces work with each other. I assume that the stock and metal parts also shrink and expand to a degree. I don't know the exact reason but I can tell you that we all had the same hitting low issue in the cold and once it warmed up, we regretted adjusting our scopes. Every shot is an opportunity to learn.

[nervoustrigger]

Hi Justin, can you hang some numbers on what you guys experienced?  I'm sure it varies quite a bit but just looking for some examples.  For example, X model rifle changed POI 1/2" at 25 yards when the temperature rose from 50F to 80F.  Something like that.

[jusanothajoe]

Temperature will affect POI on some guns more than others.
The pics below are from an HW85 in .22, I chased the shifting POI for a while before I figured out what was happening.
It didn't happen at first, it started as the weather warmed up and I was bringing the gun out of a 72 degree house out to 85-90 degree outside.
If the temp outside was the same or cooler than the house, I had very minimal shift.
I changed nothing and you can see in the pictures in order the POI shifting. 
All groups are 10 shots from 15 yards over a 20-30 minute period.
I started taking the gun outside and letting it acclimate to the outside temp and shifting stopped.

[Horatio]

Not doubting.

But the OP was specific to external ballistics with gun shot from inside at similar temperature.

[nervoustrigger]

Derrek, thanks for the example groups.  That's a remarkable change for a 20F swing.  So at 15 yards, that looks like over 1"vertical shift.  Is that about right?

Seeing that sort of thing makes me really appreciate using regulated PCPs for hunting.  I do most of my recreational shooting in the warm months when the highs are 85-95F, and in the winter I'll be outside in the early morning when it's 30F looking for gray squirrels.  Most shots are in the range of 30 to 50 yards and the change in POI is indistinguishable even after the metal has gotten so cold as to be bone chilling to the touch.  I would chalk it up to luck that the gun and scope just happen to have shifted in a complementary fashion but both the .177 and .22 with different scopes hold true to their warm weather range cards.

[Geronimo]

jason, i dont know what most everyone else was experiencing or the exact measurment but i was a half a dot low during sight in with my regulated marauder and my back up unregulated coyote. i shoot hft class so i dont know what the clickers were coming up with but i do know that most everyone i spoke with was experiencing the same trend. maybe one of the guys from another class can give better info. im still fairly new to this game and still learning. i will make an effort to recreate this scenario at home over the next week or so. pretty much the same weather pattern now as it was at the comp. mid 30s in the morning and mid 60s and 70s in the afternoon. i’ll try to get some exact measurements. i am a paper chaser in the warm months and spend every weekend in the woods chasing critters in the cold. i agree it isnt an issue when targeting food but it does make a difference when a small shift can change the your score. hope this helps and i’ll get to work on finding a better than half a dot measurement.

[Horatio]

Maybe a gun with a steel breach and steel scope tube would be less affected by temp changes?

[nervoustrigger]

Derrek, haha so we have the warm paper and cold critters thing in common.  The example of a half dot...we can actually get a good sense of how much shift that is with just a few details.  Was that at the full 55 yards and the max allowable magnification of 12x?  And that's at the 20fpe or 12fpe level?  In .22 or .177...or give the pellet?
 
You've given me some renewed interest.  This weekend we're supposed to see a drop into the mid 30's F so I'm planning to do a test to see how much shift there is, having previously zeroed in the heat of summer.  I'll report back.  I'm looking forward to your results as well.

[Geronimo]

Jason, I assume your asking me instead of Derrek. I’ve been called worst haha. the half dot reference was for my .177 marauder with 4-16x44 swat utg set at 12 power and at my zero range of 35 yards. pellet speed is 880 FPS for jsb 10.3gr or 18.5ish fpe. my coyote was basically the same though with the same make and model scope set at 12 power also zeroed at 35 yards. I always test for poi shift at my zero range first. the coyote is putting the 10.65 gr cuda match out at 915 FPS. not much difference in speed so I assume that’s why they behaved the same. my range cards are basically identical for the two. I did leave the rifles and my tanks in my truck in moms driveway so everything had cooled to outside temps overnight. she’s not a fan of guns in general and since she lets me stay at her house for the PA cup, I leave them outside. it’s that or pay for a hotel. I’d rather spend my cash on Airgun stuff haha! good luck with your testing, I look forward to seeing your results. i’ll post mine as soon as I can to compare. hunting both days this weekend so it might be a couple of days but i’ll get it done. an excuse to shoot more is all I ever need!

[jusanothajoe]

Derrek, thanks for the example groups.  That's a remarkable change for a 20F swing.  So at 15 yards, that looks like over 1"vertical shift.  Is that about right?

Seeing that sort of thing makes me really appreciate using regulated PCPs for hunting.  I do most of my recreational shooting in the warm months when the highs are 85-95F, and in the winter I'll be outside in the early morning when it's 30F looking for gray squirrels.  Most shots are in the range of 30 to 50 yards and the change in POI is indistinguishable even after the metal has gotten so cold as to be bone chilling to the touch.  I would chalk it up to luck that the gun and scope just happen to have shifted in a complementary fashion but both the .177 and .22 with different scopes hold true to their warm weather range cards.

Yes, about an 1 inch drop an 1/4 to 3/8 to the left.

[nervoustrigger]

Hi Justin, sorry yeah I meant to address that to you.  Thanks for the extra details.  My first thought is a half mildot at the 35 yard zero is more than I would have expected for a 10.3gr JSB zipping along at 880fps.  But maybe I'll get a surprise when I do my little experiment this weekend.  It just so happens that virtually every relevant factor is very similar between your field target example and my cold weather gray squirrel sessions:
 
1.  I use 12x magnification exclusively
2.  I use half dot increments for aiming precision
3.  velocity is 850fps (.177) to 900fps (.22)
4.  distances are usually 30 - 35 yards
 
Since I often use a scope cam, I have the opportunity to carefully review actual POI versus the POA.  So if there were as little as a half dot of shift, it should have been apparent...particularly considering the gray squirrel count has been substantial since I was given permission to thin out the population on a small local pecan orchard.
 
Anyway, so here's a quick description of what I'm going to do when the temperature dips this weekend.  Both rifles have recently been confirmed to be well zeroed at 43 yards in warm weather...at least 80F and probably more like 90F.  So I'm going just going to let them both soak in the cold and then see how much their POI shifts.  The temperature delta should be more like 50F so that has the opportunity to exaggerate things.
 
I'll report back!

[plinker81366]

to add, barometric, and temperature change, can affect a persons blood pressure and pulse rate

[Sfttailrdr46]

I have been shooting all my life and have learned things on this thread that was new information to this old fart 

[nervoustrigger]

Well I could not resist the urge to hit the pecan orchard on this cold morning.  I'm writing from there...just had my first shot on a gray squirrel.  Had been here for over an hour, temperature 33F, the metal ice cold.  A good 50F difference from when it was zeroed.  A 32 yard shot, reviewing the slo-mo scope cam footage showed the elevation exactly where I expected it to be. 

That's just one shot of course but it looks promising.  I still plan to put some shots on paper later.

[nervoustrigger]

Okay, here's how the trajectory held up at 45°F:



The cold front brought some unrelenting wind so I waited until it seemed to have a pretty consistent speed and then ran 5 shots pretty quick, so pay no mind to the horizontal error.  The elevation seems to be spot on.  This is at least a difference of 40°F from when it was zeroed.

[Geronimo]

Thanks for the test info nervoustrigger. Maybe we all fell victim to mass hysteria at the comp. One person blamed the cold and we all said yep, it’s the cold not me lol. I’ll do some testing to see what i come up with. I will say that i didnt have any trouble getting my limit of 6 squirrels sunday with one of my .25 marauders that i hadnt shot since mid summer. I know it’s more forgiving since a squirrel noggin is larger than a bullseye but i probably wouldve noticed that my shots were tracking low if the scope or gun had reacted negatively to the cold. I do know some of the guys saying they had problems were using springers and maybe their lubes had thickened causing slower velocities. Hard to tell without testing. At least i have a new rabbit hole to explore. Nice shootin by the way.

[nervoustrigger]

Hi Justin, yeah I was surprised a 40F swing didn't make at least some difference at that distance.  It's a Chinese gun fitted with some go-faster parts and a fair amount of elbow grease.  Nothing exotic, though.  Not even fancy lubricants.  And the scope is a Nikko-Sterling that was on sale for $80.  An excellent one for the money but nothing special there either.
 
Sounds like you had a good day yesterday, congrats on the nice haul.
 
Oh, and thanks for the attaboy on the target.  The wind was just brutal yesterday morning.  It took me a few shots to get settled in at the table and I didn't show you those   But I figured since we were mostly interested in the elevation (trajectory), I might as well give it a go so I just held on the red dots to have a consistent point of aim.  Turns out those 5 are actually sub-MoA but I certainly won't claim I can do that any time I want, even when it's calm.
 
Anyway, if you get a chance to put yours on paper, let us know how it goes!

[air max]

Your shooting from inside a warm house? The temperature difference from inside shooting outside can play a role in your problem especially if you're sticking the barrel out a window pulling it back in to cocking it then sticking it back out the window. With a spring gun i always go outside let the gun cool down wear the same jacket and gloves i hunt in and resite the gun in. I've never had my spring guns shoot very far off center. Wind can play a role if it's a true unobstructed cross wind it will move your point of impact horizontal any trees a close building can cause the wind to change directions causing it to swirl and so on. Keep shooting and have fun.