Difference in DA change from altitude vs DA change from temperature (cold powder)

Jul 4, 2017
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#1
Okay so I need some help. A few months ago I went to verify old dope for my prairie dog load and found the dope to be dead on inside 300, but at 570, it was about 0.6 mils low. The conditions were 40 degrees at about 750 ft ASL for a DA of about -340 ft. I thought it was from the polymer tips melting and dropping the BC - ammo is factory fiocchi 50 gr vmax - and everyone was extremely helpful with ideas. Then yesterday I went out to check my dope for next week's prairie dog trip to Nebraska, and my dope was dead on at 300, and at 590, I only needed 4.1 mils in 70 degrees and about 1400 ASL for a DA of about 2500. This new dope says that at 570 I would only need 3.8 mils. JBM says the DA shift should only account for 0.3 of the difference from the 4.7 in the cold vs 3.8 in the warmer weather. Is this wild swing due to the temperature of the ammo? I'm not powder expert since I don't reload and I don't know what powder fiocchi uses. I know most people use DA to figure out their adjustments, but does a 3000 ft change in DA from elevation necessarily equal a 3000 ft change in DA from mostly temperature? Any past experience with this kind of stuff would be sincerely appreciated.
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
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#2
You doubled the altitude and almost doubled the temp.
Resulting in a roughly 20% shift.
I'd guess yes.
 
Jul 4, 2017
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#3
You doubled the altitude and almost doubled the temp.
Resulting in a roughly 20% shift.
I'd guess yes.
I understand doubling the altitude, but I don't think I doubled my temperature because Fahrenheit is centered around water freezing/boiling. In terms of absolute temperature, kelvin, it's a change from 278 to 294, which is about 6%. I'm not trying to argue or anything, just understand.
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
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#4
My calculators run in f or c not k, the kiln my wife had did.
The temp is one small input in the grand sliding scale, I get that.

I got a lesson in powder temp this winter, shows why my calculator has an input for air temp and separately for powder temp.
Got all my range gear ready one day put it in my truck.
The roads started to ice up so I removed my gun and left the rest.
The next day was about 25f as I headed to the range the Ice had blow dried off mostly.

I set up and shot a couple of rounds, off damn.
70f indoor 84yd range.. I shot a couple of groups still off but warmed the barrel to ambient.
Checked everything slowed down more and shot another groop still off.

Took a break and then comenced to re zero, the damn rounds started walking.
Went home dejected, didn't even change the zero stop.
Got on the hide to vent . The guys quickly sorted out the problem.
The powder had been cold soaked overnite and then slowly warmed causing the initial shift
And subsequent walking .
Everything back to original zero the next day.
Ammo was match grade with match powder.
I will be using the powder temp function to check for changes from now on.

The powder in the rounds your using may not be very temp stable either.
For your consideration.
 
Nov 5, 2013
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#5
to answer your question about DA...the DA # is the DA #...its a factor derived from the combination of altitude, temp, pressure, and dew point...3000 is 3000

like mentioned above, the temp sensitivity of your ammo could very well prove an issue with temp swings...especially with the 50 vmax at 500+ yds
 
Likes: Snuby642
Jul 4, 2017
94
19
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#6
My calculators run in f or c not k, the kiln my wife had did.
The temp is one small input in the grand sliding scale, I get that.

I got a lesson in powder temp this winter, shows why my calculator has an input for air temp and separately for powder temp.
Got all my range gear ready one day put it in my truck.
The roads started to ice up so I removed my gun and left the rest.
The next day was about 25f as I headed to the range the Ice had blow dried off mostly.

I set up and shot a couple of rounds, off damn.
70f indoor 84yd range.. I shot a couple of groups still off but warmed the barrel to ambient.
Checked everything slowed down more and shot another groop still off.

Took a break and then comenced to re zero, the damn rounds started walking.
Went home dejected, didn't even change the zero stop.
Got on the hide to vent . The guys quickly sorted out the problem.
The powder had been cold soaked overnite and then slowly warmed causing the initial shift
And subsequent walking .
Everything back to original zero the next day.
Ammo was match grade with match powder.
I will be using the powder temp function to check for changes from now on.

The powder in the rounds your using may not be very temp stable either.
For your consideration.
I had no idea the effects could be that bad even inside 100 yds, I'll keep that in mind in the future. I normally use JBM for everything but it doesn't have a temp function so I think I'll give the Hornady app a try. Just for reference, what calculator do you use?
 

TheGerman

Oberleutnant
Jan 25, 2010
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#7
If I remember right, on the DA cards in JBM, they do not account for the temperature shift as part of a change in velocity but only as one of the variables of density altitude. Temperature is a double dip in that it effects the overall density altitude number, yet it also, separately has a direct effect on your muzzle velocity.
 
Likes: Snuby642
Jun 26, 2012
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#8
If I remember right, on the DA cards in JBM, they do not account for the temperature shift as part of a change in velocity but only as one of the variables of density altitude. Temperature is a double dip in that it effects the overall density altitude number, yet it also, separately has a direct effect on your muzzle velocity.
This is why it’s important to use the factors of DA when developing a solution instead of a straight DA number (which is good in a field expedient manner). Because temp not only affects DA it affects MV it has a profound affect on your overall firing solution.

Don’t let your ammo cook in the sun. If you do it will be hotter than ambient and you need to account for that by measuring ammo temp with a laser thermometer.
 
Jul 4, 2017
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#9
This is why it’s important to use the factors of DA when developing a solution instead of a straight DA number (which is good in a field expedient manner). Because temp not only affects DA it affects MV it has a profound affect on your overall firing solution.

Don’t let your ammo cook in the sun. If you do it will be hotter than ambient and you need to account for that by measuring ammo temp with a laser thermometer.
That make sense. I'll make sure to keep my ammo out of the sun, but I think my bigger problem is the cold. Days in the 10-35 range are all too common in Wisconsin. I'll hold on to my DA cards, and I'll have to start noting the difference on cold days between my DA adjusted dope and what I actually see. Do you use a calculator to adjust for ammo temp? If so, which one? I tried 4dof, but they don't have my bullet in their library, which isn't a huge surprise.
 
Jun 26, 2012
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#12
Between 4dof and AB you’ll do alright. I can’t recomend TRASOL right now till they get a new developer. Ballistic AE is JBM with a fancy interface.

Just remember they r tools. Don’t let them become a crutch. Nothing is better than actual dope.
 

Snuby642

Sergeant of the Hide
Feb 11, 2017
659
284
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#13
I had no idea the effects could be that bad even inside 100 yds, I'll keep that in mind in the future. I normally use JBM for everything but it doesn't have a temp function so I think I'll give the Hornady app a try. Just for reference, what calculator do you use?
My go to is sniper calc pro, it's pro version is 10$ a free version is available for evaluating.
You have to input your bullets there's no library, and limited reticles but functional.
Streyloc is also good on the cheap, same free sample version.
The one calc I did not like was Hornady.

Sniper calc and Streyloc normaly resolve within 1 click of each other so I check occasionaly.
There ar many more robust apps out there, these will work for cheap starters and use as a baseline.
 

SLG

Gunny Sergeant
Sep 2, 2009
931
322
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#14
[QUOTE="TacticalDillhole, post: 7052295, member: 71545

Just remember they r tools. Don’t let them become a crutch. Nothing is better than actual dope.[/QUOTE]

No doubt, but there are so many variables we can shoot in, and some of them are very difficult to near impossible to control or measure. Sometimes all you have is what the computer gives you. No one can develop actual dope under every circumstance and variable.
 
Jul 4, 2017
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#15
Between 4dof and AB you’ll do alright. I can’t recomend TRASOL right now till they get a new developer. Ballistic AE is JBM with a fancy interface.

Just remember they r tools. Don’t let them become a crutch. Nothing is better than actual dope.
I agree, and thankfully other than this load I've been extremely lucky with my calculators. JBM and 4dof are spot on for my 6.5 creed out to 1k, and JBM has been great for my 69 gr sierra load out to 800.
 
Jul 4, 2017
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#16
[QUOTE="TacticalDillhole, post: 7052295, member: 71545

Just remember they r tools. Don’t let them become a crutch. Nothing is better than actual dope.
No doubt, but there are so many variables we can shoot in, and some of them are very difficult to near impossible to control or measure. Sometimes all you have is what the computer gives you. No one can develop actual dope under every circumstance and variable.[/QUOTE]

My goal is to shoot in the extremes of DA and temp that I normally see - DA from -3k to +2.5k and temp from 0 to 85. Hopefully that will allow me to extrapolate for new circumstances and see where ammo temp really starts to play in to things. Down the road when I get AB, I'll play with the ammo temp input. It may not be perfect, but from 600-800 on the really cold days, I'll take anything within 0.2 or 0.3 and adjust from there.
 
Nov 5, 2013
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#17
get a magnetospeed and you can get velocity data in different temps instead of just guessing...strap it on and account for zero offset, then shoot at distance
 

Skookum

Flattus Domini
May 6, 2017
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#18
DA just confuses people, and as Tactical Dillhole said, it is more accurate just to put in absolute pressure (station pressure), temp, and leave humidity at 50%.

As far as temperature changing your powder burn rate, I think you'll find that your average change is going to fall somewhere between 0.4 fps - .07 fps per degree of ambient temperature.

Of course it will depend on which cartridge and the specifics of your load. Powders don't change much below 40 degrees F, so you won't see much change if it is colder. Above 80 degrees the change happens faster and can get pretty spikey after 95 degrees.
 
Jul 4, 2017
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#19
get a magnetospeed and you can get velocity data in different temps instead of just guessing...strap it on and account for zero offset, then shoot at distance
I would definitely do that, but in two months I’m moving to a place where I may not be able to own a rifle and I don’t think I want to spend the money right now. That’s a good idea that I should have thought of though, thanks.
 
Jul 4, 2017
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#20
DA just confuses people, and as Tactical Dillhole said, it is more accurate just to put in absolute pressure (station pressure), temp, and leave humidity at 50%.

As far as temperature changing your powder burn rate, I think you'll find that your average change is going to fall somewhere between 0.4 fps - .07 fps per degree of ambient temperature.

Of course it will depend on which cartridge and the specifics of your load. Powders don't change much below 40 degrees F, so you won't see much change if it is colder. Above 80 degrees the change happens faster and can get pretty spikey after 95 degrees.
Thankfully most of the shooting I do is between 40 and 80, so that rule of thumb should work fairly well for me for the time being. And here I thought I had figured it out with DA, everything being rolled into one number, but I guess not.
 

SLG

Gunny Sergeant
Sep 2, 2009
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#21
Don’t let your ammo cook in the sun. If you do it will be hotter than ambient and you need to account for that by measuring ammo temp with a laser thermometer.
Curious to hear how you or others keep their ammo from cooking during hot days in the field. Do you just keep it out of direct sunlight, or what?

Mine is either in my pack or in a pouch, but I often wonder how hot it gets in there.
 

Skookum

Flattus Domini
May 6, 2017
835
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Your mom's
#22
Thankfully most of the shooting I do is between 40 and 80, so that rule of thumb should work fairly well for me for the time being. And here I thought I had figured it out with DA, everything being rolled into one number, but I guess not.
Density Altitude is appropriate for fuel burning machines because Temperature, Humidity, and Pressure (more pressure, more available oxygen) affect combustion and therefore power output. And those things all act together in proportion to one another. That is why you can get the same DA number with different combinations.

Those three things act on a cartridge and a bullet in flight very differently. A bullet in flight cares absolutely nothing about temperature, it only cares how thick the air is that it is flying through. Temperature and humidity are somewhat responsible for determining that density, of course, but altitude is much more of a determining factor than either of those.

Humidity means almost nothing for our purposes, but temperature has a major affect on powder burn rate and therefore velocity. So temperature affects our shot in two completely unrelated ways, one minor and one major. The minor is accounted for when you put in your station pressure and the other is accounted for when you account for the change in powder burn rate.
 

Skookum

Flattus Domini
May 6, 2017
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#23
Curious to hear how you or others keep their ammo from cooking during hot days in the field. Do you just keep it out of direct sunlight, or what?

Mine is either in my pack or in a pouch, but I often wonder how hot it gets in there.
I want my ammo to be at ambient temp so I can figure the change in powder burn rate. I just don't want it cooking in the sun, so I keep it shaded, the same for my rifle.

The one thing I haven't figured out how to test yet is temperature affect on primers. If anyone knows of a printed source I'd love to read it.
 
Jun 26, 2012
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#24
Density Altitude is appropriate for fuel burning machines because Temperature, Humidity, and Pressure (more pressure, more available oxygen) affect combustion and therefore power output. And those things all act together in proportion to one another. That is why you can get the same DA number with different combinations.

Those three things act on a cartridge and a bullet in flight very differently. A bullet in flight cares absolutely nothing about temperature, it only cares how thick the air is that it is flying through. Temperature and humidity are somewhat responsible for determining that density, of course, but altitude is much more of a determining factor than either of those.

Humidity means almost nothing for our purposes, but temperature has a major affect on powder burn rate and therefore velocity. So temperature affects our shot in two completely unrelated ways, one minor and one major. The minor is accounted for when you put in your station pressure and the other is accounted for when you account for the change in powder burn rate.
Actually, temperature is much more a factor in DA than “altitude/elevation”. Think about it, if I’m in Denver at 5000 ft and my station pressure is 24.92 and current temp is 41F, what’s my DA? 5000 ft. But if I have the same pressure and increase or decrease my temp, my DA will change. Now of course if I go up or down in elevation my pressure will change but on any given day at any given location, it’s temp that will affect your DA. You can have a 30-40 degree temp changes in a given day, it’s pretty rare the pressure will change drastically enough to affect DA. Every 15 degrees will increase or decrease your DA by 1000 ft (approximately) it takes a 1” Mercury change to do the same thing and if your barometer is increasing or decreasing by 1” over a few hours or even a day, I suggest you seek shelter quickly as something bad is gonna happen weather wise.

This is all assuming you are in a fixed area like your range. If your hunting animals or humans and going up and down mountain passes and such yeah pressure carries equal weight as temp. But I can show up at the range at 8am, get my station pressure and never check it again all day. By just monitoring my temp I can be within 250 feet of my actual DA. That’s more than accurate enough for what we do. Unless a storm is brewing your pressure will not drop by .50” in Hg in a given day without having huge temperature swings to go with it.

DA = Station Pressure corrected for non standard temp. It’s not temperature corrected for non standard station pressure. Even if I never took a pressure reading on the above day in Denver and the temp was 85 degrees, I know that’s roughly 45 degrees above standard for that area and my DA will be somewhere in the ballpark of 8000 ft.

We are discussing semantics of course here, truth be told they are equally important, pressure and temp that is as I totally agree on humidity to set at 50% and never change it. We typically shoot from fixed locations which makes pressure relatively fixed but temp will change thus changing your DA. If you change locations higher or lower, pressure and temp will change but once established in that new AO, it’s temp that will cause the changes, pressure again will remain relatively steady.

Here is how i set up my field expedient drop sheets:

Untitled.jpg

i get my initial pressure reading and shoose the sheet closest to that pressure, then i find the current temp column and go down to find my range and get my drop. as temp changes i enter the appropriate temp column, but i rarely ever have to change sheets due to a pressure change. The circled data above is MV, DA, Temp respectively.
 

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Skookum

Flattus Domini
May 6, 2017
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Your mom's
#25
Actually, temperature is much more a factor in DA than “altitude/elevation”. Think about it, if I’m in Denver at 5000 ft and my station pressure is 24.92 and current temp is 41F, what’s my DA? 5000 ft. But if I have the same pressure and increase or decrease my temp, my DA will change. Now of course if I go up or down in elevation my pressure will change but on any given day at any given location, it’s temp that will affect your DA. You can have a 30-40 degree temp changes in a given day, it’s pretty rare the pressure will change drastically enough to affect DA. Every 15 degrees will increase or decrease your DA by 1000 ft (approximately) it takes a 1” Mercury change to do the same thing and if your barometer is increasing or decreasing by 1” over a few hours or even a day, I suggest you seek shelter quickly as something bad is gonna happen weather wise.

This is all assuming you are in a fixed area like your range. If your hunting animals or humans and going up and down mountain passes and such yeah pressure carries equal weight as temp. But I can show up at the range at 8am, get my station pressure and never check it again all day. By just monitoring my temp I can be within 250 feet of my actual DA. That’s more than accurate enough for what we do. Unless a storm is brewing your pressure will not drop by .50” in Hg in a given day without having huge temperature swings to go with it.

DA = Station Pressure corrected for non standard temp. It’s not temperature corrected for non standard station pressure. Even if I never took a pressure reading on the above day in Denver and the temp was 85 degrees, I know that’s roughly 45 degrees above standard for that area and my DA will be somewhere in the ballpark of 8000 ft.

We are discussing semantics of course here, truth be told they are equally important, pressure and temp that is as I totally agree on humidity to set at 50% and never change it. We typically shoot from fixed locations which makes pressure relatively fixed but temp will change thus changing your DA. If you change locations higher or lower, pressure and temp will change but once established in that new AO, it’s temp that will cause the changes, pressure again will remain relatively steady.

Here is how i set up my field expedient drop sheets:

View attachment 6905050

i get my initial pressure reading and shoose the sheet closest to that pressure, then i find the current temp column and go down to find my range and get my drop. as temp changes i enter the appropriate temp column, but i rarely ever have to change sheets due to a pressure change. The circled data above is MV, DA, Temp respectively.
That is all great info, but I believe you missed the point I was trying to make. My point was that altitude affects the STATION PRESSURE much more rapidly, and therefore trajectory.

At any given altitude, from winter to summer, the station pressure might fluctuate from standard pressure 0.5Hg either way. So, basically equivalent to a 1,000 ft elevation change. But on the same day, covering varied terrain, you could make an equal change by just climbing to the top of a ridge. That is what I was getting at.
 
Jun 26, 2012
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#26
That is all great info, but I believe you missed the point I was trying to make. My point was that altitude affects the STATION PRESSURE much more rapidly, and therefore trajectory.

At any given altitude, from winter to summer, the station pressure might fluctuate from standard pressure 0.5Hg either way. So, basically equivalent to a 1,000 ft elevation change. But on the same day, covering varied terrain, you could make an equal change by just climbing to the top of a ridge. That is what I was getting at.
Ok that I agree with.
 
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