Interesting Barrel heat / POI observation

[JDM9691]

Hi all, Inspired a bit my MarkandSam, I have been wanting to do some longer range stuff with my .243 using 87g vmax.

Finally got some calm weather a couple mornings back and went out on my property before 0700. Took obs on temp (10c) and RH (82) when I started, and plugged those values into the scope and Hornady app. Did a 3 shot zero at 100m which was 28mm high and .57MOA, and adjusted to be bang on.

I then left the rifle in the bags, and drove down range to setup some targets at 800 and 1000m. After driving back, I did some more obs and entered them into the scope and app. The sun was out and it had warmed to 16C and 60%rh, with 1-2k of wind. By the time I took the first shot maybe 10-15 minutes had gone by. After 3 shots at the 800m target I went downrange to look and was mystified when the .8x2m sheet of corrugated iron had no holes in it, but I could see some impact marks on the ground behind and to the left, estimating that I had missed the aim point by around 7-800mm! How could that be possible? The rifle and ammo are usually sub 2/3 moa from bags, after careful zeroing the rifle had not moved, or been bumped, the scope's BC drop value pretty much agreed with the hornady app, and the slight wind would have taken the bullets the opposite way Yet some how, the POI had shifted around 4 MOA to the left. I was stumped.

I drove back to the rifle and pondered some more. When I picked the rifle up, a lightbulb lit up. I could feel that the right hand side of the black poly stock was warm from catching the early morning sun square on, and the left hand side was cool. Sure enough, the same was true with the barrel, and my theory is that the sun heating and expanding the right hand side was enough to give it a tiny bend to the left. After allowing time to cool in the shade, it was then shooting to the right, so it would appear the initial zero was affected by the sun as well. I gave up where the zero was, and just allowed for the next shot based on where the previous one hit, and was fortunate enough to hit the 1000m gong after 2 shots, which made up for all the missing at 800

So I've learned something which I thought I'd share. Maybe this is nothing new for you guys, but I've read and watched a lot of stuff on the internet about barrel heat and POI shift, and don't remember anyone attributing it to sunlight. Perhaps they shoot from the shade, which I'll be doing in future. Anyone else run into this sort of thing before?

[zbenga]

can't be that much, maybe 1/4 MOA with a really really thin barrel and say 20 degree temp diffrence

maybe be the crown is cut bad and that is the issue even with small temp differences in the barrel or maybe the bedding is bad and very susceptible to temp shift?

generally I don't even adjust for temp, only humidity, distance and wind

[deye243]

What did you put in for ballistic coefficient and what device are you using for measuring bullet speed and scope height above bore

[bladeracer]

Hi all, Inspired a bit my MarkandSam, I have been wanting to do some longer range stuff with my .243 using 87g vmax.

Finally got some calm weather a couple mornings back and went out on my property before 0700. Took obs on temp (10c) and RH (82) when I started, and plugged those values into the scope and Hornady app. Did a 3 shot zero at 100m which was 28mm high and .57MOA, and adjusted to be bang on.

I then left the rifle in the bags, and drove down range to setup some targets at 800 and 1000m. After driving back, I did some more obs and entered them into the scope and app. The sun was out and it had warmed to 16C and 60%rh, with 1-2k of wind. By the time I took the first shot maybe 10-15 minutes had gone by. After 3 shots at the 800m target I went downrange to look and was mystified when the .8x2m sheet of corrugated iron had no holes in it, but I could see some impact marks on the ground behind and to the left, estimating that I had missed the aim point by around 7-800mm! How could that be possible? The rifle and ammo are usually sub 2/3 moa from bags, after careful zeroing the rifle had not moved, or been bumped, the scope's BC drop value pretty much agreed with the hornady app, and the slight wind would have taken the bullets the opposite way Yet some how, the POI had shifted around 4 MOA to the left. I was stumped.

I drove back to the rifle and pondered some more. When I picked the rifle up, a lightbulb lit up. I could feel that the right hand side of the black poly stock was warm from catching the early morning sun square on, and the left hand side was cool. Sure enough, the same was true with the barrel, and my theory is that the sun heating and expanding the right hand side was enough to give it a tiny bend to the left. After allowing time to cool in the shade, it was then shooting to the right, so it would appear the initial zero was affected by the sun as well. I gave up where the zero was, and just allowed for the next shot based on where the previous one hit, and was fortunate enough to hit the 1000m gong after 2 shots, which made up for all the missing at 800

So I've learned something which I thought I'd share. Maybe this is nothing new for you guys, but I've read and watched a lot of stuff on the internet about barrel heat and POI shift, and don't remember anyone attributing it to sunlight. Perhaps they shoot from the shade, which I'll be doing in future. Anyone else run into this sort of thing before?

Is any of your line of sight within two or three meters of the ground? Could it simply be mirage as the temperature rose? Mirage will let you still see the target, but it's not where it appears to be due to light being refracted by the heat waves rising.

One spot where I shoot long-range .22LR it's best to shoot before the sun gets too high, once the ground warms up I have no hope of hitting the targets because of the mirage. I can still see the targets just fine, but the targets are no longer where my bullets are going.

[Wapiti]

Mate it is VERY possible that the sun on one side of a barrel will add to the heat of shooting, even without, to walk the POA.
If the barrel has any inbuilt stresses, which most do, it will definitely walk it's shots.
Even without stresses built in in manufacture or straightening, which is normal to do if needed.

Boilermakers will tell you that an everyday observation to temporarily bend any steel section in any direction is the application of heat on one side. Think bridge sections, truck-trailer set bends, it's normal to do this. Barrel steel with it's typical dimensions is a perfect example of this.

Most people have no idea of this, even gun-writers in magazines and internet when trying to justify why a rifle inexplicably walks it's shots a few inches or more on a target. I laugh at the reasons given sometimes..

So,
If the barrel has any inbuilt tension in it (which is quite common but not always) on one side of it's circumference, when heat is applied it will walk in that direction the amount the stress allows it, where it will stop until the stress cannot move further, or;
If heat is applied on one side, such as from the sun, until the heat is fully absorbed through the barrels circumference, the barrel will imperceptively change shape away from the heated side... remember the heated side expands and gets longer temporarily. Because the heat applied is not at or above the temperature needed to change the shape permanently, when it cools the barrel will come back.

A barrel with a good bit of stress in it, which is quite common, will throw it's first shot when cool to a different spot. In many cases, that "cold-bore" shot is due to that, it's not always a "clean bore" reason. The huge heat energy of just one shot can cause a barrel to move to the spot it likes and stay there for the rest of the group.
Others will keep walking as the barrel heats up more each shot.
Stress in steel.

[zbenga]

I think 4moa is a lot for a cold bore shot no? mine is half MOA and on all my rifles the cold shot is half MOA to the top left, every bloody time, all my barrels are cold hammer forged, maybe the spun ally ones are different?

I think bladeracer is right, the mirage can sometimes do funny things, specially early in the morning if you shoot targets close to ground and the grass was wet then sun comes up strongly you get this mirage which does not move , it's more like it shimmers up, every time I have had that the POI is super random, even follow up shots are 2moa out

[Wapiti]

All I was doing was answering the question as to whether heat on one side of a barrel can move it, solely on your observations of that occurrence.
The amount a piece of steel moves from the sun's heat is pretty surprising, and most of us know how much a tiny bit of influence touching a barrel makes, resting it on something, a stock slightly pushing on one side, whatever, makes.
I never see this ever discussed, I haven't anyway, and it makes more than a measurable difference.

What actually caused it in your case is what you have to eliminate to fix it.

[straightshooter]

Interesting theories.
Has the original poster ever heard of wind?

[zbenga]

All I was doing was answering the question as to whether heat on one side of a barrel can move it, solely on your observations of that occurrence.
The amount a piece of steel moves from the sun's heat is pretty surprising, and most of us know how much a tiny bit of influence touching a barrel makes, resting it on something, a stock slightly pushing on one side, whatever, makes.
I never see this ever discussed, I haven't anyway, and it makes more than a measurable difference.

What actually caused it in your case is what you have to eliminate to fix it.

I think the biggest issue would be where the crown hits the rifling ... if the rifling ends around that part that got heated then it can do some impressive POI


I think to move the POI by 4 MOA you need some wind sure the OP would of picked that up

[CJRBOLTGUN1]

Gunna get flamed for this but here goes,

4DOF sucks ,Don't get me wrong it's simple and portable but, I personally found that distances up to 400m it was fairly good but at 500m things got drastically out of whack ,tried running the data again and same ,the app kept telling me to dial/hold 3.6mrad but in reality I only needed 2.5 ,now I get that it's not going to be bang on but that's way off, so I went back to using the simple free calc that's available on the Webb and found it is a lot closer to the real numbers when getting out past 400m,let the flaming begin..................lol

[bladeracer]

Gunna get flamed for this but here goes,

4DOF sucks ,Don't get me wrong it's simple and portable but, I personally found that distances up to 400m it was fairly good but at 500m things got drastically out of whack ,tried running the data again and same ,the app kept telling me to dial/hold 3.6mrad but in reality I only needed 2.5 ,now I get that it's not going to be bang on but that's way off, so I went back to using the simple free calc that's available on the Webb and found it is a lot closer to the real numbers when getting out past 400m,let the flaming begin..................lol

I've been using the online shooterscalculator.com for years now and it's been excellent.
BC decreases with velocity (distance). I assume that most good BC calculators already take into account this effect. Sierra is the only manufacturer I'm aware of that publishes different BC's based on bullet velocity. For example, their 55gn SBK is .271 above 3100fps, drops to .264 under 3100fps, drops to .250 under 2550fps, .236 under 2550fps, and is only .224 under 2250fps. These aren't "walls", these are just measurement points along the gradual loss of BC as velocity bleeds off. I found the .22LR BC is good to around 330m (700fps), then it starts reducing fairly rapidly.

Regardless of published BC's it's worth measuring your own BC if possible. The simplest way I've found is via velocities at several distances, but to be precise you need lots of data points or very tight ES. If you fire twenty rounds at 200m and the ES is 50fps, but you have ten shots that have ES of 8fps, discard the outliers and just average the tight ones or you'll have no hope of tweaking the BC to match the velocity curve. With Eley Standard I might have velocities from 700fps up to 815fps, but a bunch would be in the 790fps to 805fps window, so I would only use those to calculate the mean. There's no point including the velocity outliers as those are going to be fliers in a group anyway.

[CJRBOLTGUN1]

Gunna get flamed for this but here goes,

4DOF sucks ,Don't get me wrong it's simple and portable but, I personally found that distances up to 400m it was fairly good but at 500m things got drastically out of whack ,tried running the data again and same ,the app kept telling me to dial/hold 3.6mrad but in reality I only needed 2.5 ,now I get that it's not going to be bang on but that's way off, so I went back to using the simple free calc that's available on the Webb and found it is a lot closer to the real numbers when getting out past 400m,let the flaming begin..................lol

I've been using the online shooterscalculator.com for years now and it's been excellent.
BC decreases with velocity (distance). I assume that most good BC calculators already take into account this effect. Sierra is the only manufacturer I'm aware of that publishes different BC's based on bullet velocity. For example, their 55gn SBK is .271 above 3100fps, drops to .264 under 3100fps, drops to .250 under 2550fps, .236 under 2550fps, and is only .224 under 2250fps. These aren't "walls", these are just measurement points along the gradual loss of BC as velocity bleeds off. I found the .22LR BC is good to around 330m (700fps), then it starts reducing fairly rapidly.

Regardless of published BC's it's worth measuring your own BC if possible. The simplest way I've found is via velocities at several distances, but to be precise you need lots of data points or very tight ES. If you fire twenty rounds at 200m and the ES is 50fps, but you have ten shots that have ES of 8fps, discard the outliers and just average the tight ones or you'll have no hope of tweaking the BC to match the velocity curve. With Eley Standard I might have velocities from 700fps up to 815fps, but a bunch would be in the 790fps to 805fps window, so I would only use those to calculate the mean. There's no point including the velocity outliers as those are going to be fliers in a group anyway.

yes, that's the one that I use too ,

the bc factor for some guys is all about bragging writes, well that's the impression you get when you ask them to explain how that number is derived, most will just say that's what's on the box and a lot will quote the box velocity like its written in stone even if there rifle has a different barrel length ,I usually just switch off and go about my business and have a chuckle to myself

[bladeracer]

yes, that's the one that I use too ,

the bc factor for some guys is all about bragging writes, well that's the impression you get when you ask them to explain how that number is derived, most will just say that's what's on the box and a lot will quote the box velocity like its written in stone even if there rifle has a different barrel length ,I usually just switch off and go about my business and have a chuckle to myself

I don't know how BC's were calculated in the past but all I need to know about a bullet's BC is how quickly it sheds velocity over distance, essentially, how much drag it has. Now that chronographs are cheap enough that anybody can own one, taking velocity measurements at several distances is pretty easy to do (though I have shot a chronograph doing this). Along with measuring bullet drop at several distances you can calculate for yourself a fairly close BC number to use in ballistic calculations.

[CJRBOLTGUN1]

yes, that's the one that I use too ,

the bc factor for some guys is all about bragging writes, well that's the impression you get when you ask them to explain how that number is derived, most will just say that's what's on the box and a lot will quote the box velocity like its written in stone even if there rifle has a different barrel length ,I usually just switch off and go about my business and have a chuckle to myself

I don't know how BC's were calculated in the past but all I need to know about a bullet's BC is how quickly it sheds velocity over distance, essentially, how much drag it has. Now that chronographs are cheap enough that anybody can own one, taking velocity measurements at several distances is pretty easy to do (though I have shot a chronograph doing this). Along with measuring bullet drop at several distances you can calculate for yourself a fairly close BC number to use in ballistic calculations.

probably done the way you've described,
personally think Chronos have gotton more expensive ,I've had several since the learly 2000's and they didn't cost half as much as todays Chronos that are available to the public, but they were susceptible to lead poisoning: silent:

[bladeracer]

probably done the way you've described,
personally think Chronos have gotten more expensive ,I've had several since the learly 2000's and they didn't cost half as much as todays Chronos that are available to the public, but they were susceptible to lead poisoning: silent:

Cheap chronographs are still available, but there are now much, much more expensive options available as well. I don't have any need for a $1000 chrono, my $300 ones have all worked just fine, and I'm not going to cry when it dies or I shoot it (again). I shoot slowly over the chrono to ensure I get clear measurements, but my mate dumped a mag from his GSG1911 .22 over it very rapid fire and it caught all ten shots. I was impressed - I expected to see maybe the first reading and some errors.

I used to set the chrono up almost every time I was shooting just to have the additional data recorded. I recorded many, many thousands of shots. I did very little with the data though and soon realised it was wasted effort. The chrono lives in the car but I just use it when I actually want velocity data now, and it's handy to have it on-hand when somebody else wants data for themselves (the tripod is also handy for setting up the camera when shooting). When you're putting in the dollars and time to do a serious ammunition test then it makes sense to gain as much data as possible, even if you don't end up needing it, but for working up a load, you don't need a lot of velocity measurements.

[Fester]

I wish the new type chronos were getting cheaper, as after seeing how easy they are to use, I just can't be assed setting up the old Caldwell anymore.

Ron Spomer, who loves using the ballistic apps for his content, did an episode on calculating any one missing data figure.
It all made sense and wasn't complicated.
I don't remember if the BC could be found or not.

We plebs have no other real way to get BCs than the company claims and box writings.

I just never gave that sun on one side a thought, apart from doing my tyre pressures on the driveway.

[bladeracer]

I wish the new type chronos were getting cheaper, as after seeing how easy they are to use, I just can't be assed setting up the old Caldwell anymore.

Ron Spomer, who loves using the ballistic apps for his content, did an episode on calculating any one missing data figure.
It all made sense and wasn't complicated.
I don't remember if the BC could be found or not.

We plebs have no other real way to get BCs than the company claims and box writings.

I just never gave that sun on one side a thought, apart from doing my tyre pressures on the driveway.

If the firearm is fairly accurate, ie it shoots smallish consistent groups, you can get decently close to the BC simply by plotting the trajectory of groups shot at different distances. If it's less accurate you can still do it but will require lots of data points at each distance so the average bullet drop can be measured. Then it's a matter of tweaking the numbers (estimated velocity and BC) in a ballistic calculator until you have a calculated trajectory that matches your real trajectory.

[JDM9691]

What did you put in for ballistic coefficient and what device are you using for measuring bullet speed and scope height above bore

.400 BC, no MV measurement as I don't have the equipment, but using GRT gives me a reasonable estimation, Scope height measured at the breech with a steel rule. Should be within a mm or two, but i'm open to any better method if you have one!

[JDM9691]

Interesting theories.
Has the original poster ever heard of wind?

Yep, that's why I was out doing it so early in the morning - I wanted to test my shooting skills, not my wind reading skills.
I didn't mention wind because there was barely any at the time, if anything it was 1-2 kmph slightly left to right, and the impact was 4MOA to the left, so as far as I was concerned, the wind wasn't a variable.

[JDM9691]

Is any of your line of sight within two or three meters of the ground? Could it simply be mirage as the temperature rose? Mirage will let you still see the target, but it's not where it appears to be due to light being refracted by the heat waves rising.

Yes, I'm shooting across a slight gully. From memory there was a slight mirage in the scope during the long shots, that wasn't there in the 100m zero. I was out early hoping to avoid wind and mirage, but there certainly was some there before 8 a.m.

[JDM9691]

If the firearm is fairly accurate, ie it shoots smallish consistent groups, you can get decently close to the BC simply by plotting the trajectory of groups shot at different distances. If it's less accurate you can still do it but will require lots of data points at each distance so the average bullet drop can be measured. Then it's a matter of tweaking the numbers (estimated velocity and BC) in a ballistic calculator until you have a calculated trajectory that matches your real trajectory.

That was part of this little exercise - to gather some DOPE and tweak the BC/MV using some calculators. The vertical results at long range were overshadowed by the horizontal, but when I hypothesized sun heat on one side of the barrel, I also figured that because the sun was above the horizon, then any barrel warping would be pushing the muzzle down slightly as well as left, so I didn't take it any further. I've been too busy to get back out and have another go, but will the next chance I get.

[JDM9691]

Gunna get flamed for this but here goes,

4DOF sucks ,Don't get me wrong it's simple and portable but, I personally found that distances up to 400m it was fairly good but at 500m things got drastically out of whack ,tried running the data again and same ,the app kept telling me to dial/hold 3.6mrad but in reality I only needed 2.5 ,now I get that it's not going to be bang on but that's way off, so I went back to using the simple free calc that's available on the Webb and found it is a lot closer to the real numbers when getting out past 400m,let the flaming begin..................lol

I'm only using the Hornady G1/G7 as the 4DOF doesn't list my projectiles. I was also using the Ballistic Calculator on the Alpex4k to get drop values, and comparing that to the app, hoping to gather a bit of data and measure some actual values. The Alpex doesn't take into account ATM pressure or humidity, so those two were never going to agree anyway. What online calculator were you using?

[CJRBOLTGUN1]

Gunna get flamed for this but here goes,

4DOF sucks ,Don't get me wrong it's simple and portable but, I personally found that distances up to 400m it was fairly good but at 500m things got drastically out of whack ,tried running the data again and same ,the app kept telling me to dial/hold 3.6mrad but in reality I only needed 2.5 ,now I get that it's not going to be bang on but that's way off, so I went back to using the simple free calc that's available on the Webb and found it is a lot closer to the real numbers when getting out past 400m,let the flaming begin..................lol

I'm only using the Hornady G1/G7 as the 4DOF doesn't list my projectiles. I was also using the Ballistic Calculator on the Alpex4k to get drop values, and comparing that to the app, hoping to gather a bit of data and measure some actual values. The Alpex doesn't take into account ATM pressure or humidity, so those two were never going to agree anyway. What online calculator were you using?

Ive tried a few different ones but i keep going back to SHOOTERSCALCULATOR.COM,its simple to use and free but it gets me the closest on target

https://shooterscalculator.com/

[JDM9691]

Thanks for posting. It looks easy to use, but I cant handle having to convert everything to inches, yards, MPH and Degrees F.
I did have this one bookmarked, which allows different units for input and output. Results aren't far off the hornady app
https://jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

[bladeracer]

Thanks for posting. It looks easy to use, but I cant handle having to convert everything to inches, yards, MPH and Degrees F.
I did have this one bookmarked, which allows different units for input and output. Results aren't far off the hornady app
https://jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

Try this link as this is from my bookmarks. I added meters for distance and cm for wind and drop. Distance inputs are still in yards as I don't know how to change that, but the difference is essentially ten-percent. 300m is roughly 330yd, 300yd is roughly 275m. If you want to zero at 100m input 110yd. I don't bother with the environmental factors as the differences are tiny, smaller than wind effects.

https://shooterscalculator.com/ballistic-trajectory-chart.php?pl=%5BPreset+Name%5D&presets=&df=G1&bc=0.135&bw=40&vi=1070&zr=110&sh=3.145&sa=0&ws=10&wa=90&ssb=on&cr=550&ss=1&chartColumns=Range~m%60Elevation~cm%60Elevation~MOA~FBFFF5%60Windage~cm%60Windage~MOA~FBFFF5%60Time~s%60Energy~ft.lbf%60Vel%5Bx%2By%5D~ft%2Fs&lbl=&submitst=+Create+Graph+

[CJRBOLTGUN1]

Thanks for posting. It looks easy to use, but I cant handle having to convert everything to inches, yards, MPH and Degrees F.
I did have this one bookmarked, which allows different units for input and output. Results aren't far off the hornady app
https://jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

yeah, I use that as well, Dunno every rifle I've tried with 4dof it's good up to 400m then it doesn't seem to give me the numbers I need, maybe it's me: roll: