# Warm weather vs. cold weather trajectory



## Plainsman (Jul 30, 2003)

I do most of my rifle trajectory tables to coincide with the hunting I do. I do my choreographing and program for my 300 mag in 20 degree weather for deer season, my single shot 22-250 at 80 degrees for shooting prairie dogs etc. I use 0 degrees to set up my coyote rig. When I miss something I think the program is wrong or the rifle is dirty, or some other reason. I assumed that temperature made some difference, and I also assumed that Hogdon extreme powders would show less variation. So with all this in mind here is something for you to chew on.
My 308 700P with a Lapua Silver Scenar 155 gr and 47 gr Varget
80 degrees it chronographs 2940 fps
70 degrees 2930 fps
30 degrees 2840 fps
0 degrees 2810 fps

So here is the problem. Computer programs say that at 800 yards there should only be 16 inches difference in drop. Real shooting produces a 32 inch difference in drop.

Factory Black Hills match using 168 gr Sierra Match King does produce 16 inches difference in drop, but I have not done enough choreographing of this round to match up temperature and drop differences in a trajectory program. Don't misinterpret this statement, the Lapua is still dropping 32 inches less than the Sierra Match King.

This rifle load produces the same results with a clean or dirty barrel. Accuracy does not suffer, and groups are consistently under .3 inches. On Sunday with two different shooters all shots were within a 4 inch circle at 800 yards. I can not reconcile the calculated vs. real world shooting trajectory differences.

I don't see this great of a variation in any of my other rifles. What do you guys think could be the explanation. Once velocities are established the type of powder is out or the equation. The ballistic coefficient of .508 provided by the factory is my only suspect. I say this because if I enter equal velocities for the SMK the trajectory is near identical although the ballistic coefficient for the 168 SMK is only .462.


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## Jiffy (Apr 22, 2005)

Damn Norsks!! You can't freaking trust 'em!! Giving out incorrect data and all!! :wink: :lol:

The only logical explantion sounds like you must have enter in some incorrect data. What? You got me but I'll take "real world" data over something computer generated anyday.

HEY.......I know, maybe it was magic!


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## Plainsman (Jul 30, 2003)

In the Sierra program you pick your bullet and all the bullet data is entered for you. You fill in velocity, barometric pressure, elevation, all the exterior variables. 
I have run this program and others (using the .508 ballistic coefficient) and programs don't match the real world. That's no real revelation, what confuses me is the huge change with temperature variables. I can live with it by adding 1/2 inch per hundred hards for cold weather, but I want to know why it's happening.


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## Gohon (Feb 14, 2005)

I get lost real fast in these kind of discussions and my comprehension of your post is also very limited. Here is something I found and marked awhile back. Is it possible the threshold of radical BC changes as explained below are being met sooner in colder denser air? Like I said my knowledge of this subject is very limited so I apologize if sounding dumb.

"The ballistic coefficient of each bullet changes with velocity of the bullet as it flies. The ballistic coefficient of a bullet is not constant with bullet velocity. The reason that the BC changes with velocity is that the standard drag function (the G1 drag function) does not characterize the aerodynamic drag on any sporting bullet throughout the full range of its velocity from the gun muzzle to impact. When a bullet is fired with a supersonic muzzle velocity, as its velocity falls there can be a gradual change in ballistic coefficient until the bullet reaches a velocity near 1600 fps (which is in the upper transonic velocity region). When the bullet velocity falls below 1600 fps, radical changes in ballistic coefficient begin to occur.

When the bullet velocity is greater than 1600 fps, the G1 drag function is a reasonable model from which to compute the aerodynamic drag on a bullet. The gradual changes in BC value with velocity can be handled in trajectory calculations by adjusting the BC values used in those calculations by changing the BC stepwise as the bullet traverses four or five velocity regions. The trajectory will start with the bullet velocity in one of those velocity regions. As the bullet velocity decreases and crosses the boundary between that initial velocity region and the next lower region, the BC is changed to the value corresponding to the next lower region. This process is repeated as the bullet velocity falls through successively lower velocity regions".


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## Plainsman (Jul 30, 2003)

Thanks Gohon. My program does enter four different ballistic coefficients starting with the standard advertised ballistic coefficient and adjusting for velocities. 
I started using the 155 gr Scenar because it remains supersonic to 1400 yards, while the 168 gr SMK goes subsonic at somewhere below 1000 yards. The odd thing is, my load appears to have a greater verticle variability than the Black Hills match ammo. A higher ballistic coefficient should result in less variability. None the less, I think the variability has to be in the flight/velocity characteristics, as affected by temperature. Still scratching my head on this one.


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## neb_bo (Feb 3, 2007)

did you consider relative humidity? i know this can make more of a difference than you might think, especialy at the longer ranges. i deffinetely dont have the technical knowledge you do, but after reading this is the only thing i could think of.


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## Horsager (Aug 31, 2006)

My gut tells me it's a combination of denser air and humidity. This is a silly question but do you have the altitude entered correctly on your ballistics program?


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## Plainsman (Jul 30, 2003)

I round of the altitude for hunting here in North Dakota. Around Jamestown we are at 1350, and back home we are 1550, so I use 1500 as my elevation. When I go to Montana elk hunting I have two programs made up,one at 6000 for camp and one at 8000ft for the mountain I hunt on.

I have entered current barometric pressure and gone to the range within minutes. That doesn't change anything. I suspected my scope, but at 100 yards everything is consistent. I have fired factory rounds as a test control and that turns out as my programs indicate.

The programs do not predict the difference I am seeing in real world shooting. It's frustrating, but I guess I will have to look at the bright side. I will get to do more shooting, and I will find out what is going on. One good thing about this: is there anything that looks nicer than 1000 Lapua Scenars coming together with Black Hills Match brass, Federal Match primers, and Hogdon Varget. And people don't think hunters are romantic. 

[siteimg]6650[/siteimg]


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## Horsager (Aug 31, 2006)

As you change your temp/pressure in your ballistics program do you suppose that it attempts to account for what happens regarding variations in burn rate "internally" or air density "externally"?

Are you by chance shooting over water or a wetland that would be cooler and more moist than the surrounding conditions?

Any idea over what distance the B.C. of the Lupua bullet is measured? Barnes recently re-stated a bunch of B.C.'s for their TSX's after doing the measurements at both 100 and 300 yds instead of just 100 as they started out with. Another obvious question would be did your run your numbers through more than one ballistics program?

Just a few thoughts.


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## xdeano (Jan 14, 2005)

> Are you by chance shooting over water or a wetland that would be cooler and more moist than the surrounding conditions?


That might just be it Plainsman. If i know where you're shooting he's head on. That sounds like a good explenation.

xdeano[/quote]


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## Plainsman (Jul 30, 2003)

I am shooting two places, one over water, one not. However, variations occur both places at equal rates. Also, I run the program with 20 percent and 90 percent humidity. This makes less than one inch difference at 800 yards. The 100 yard holdover for 800 yards is 19.27 at 20 percent humidity, and 19.19 for 90 percent humidity. That would be .08 inches difference times 8 for only .64 inch difference at 800 yards. 
The Sierra program has four ballistic coefficients for the Scenar, but I don't know what the ranges are. I would guess 100, 200, 300, and 400 yards.
I have been using Sierra, and two online programs. The programs are spot on for all but loads with the Scenars. 
Xdeano, if you get a chance to shoot at different temperatures at long range see what yours are doing.

As I said there is no problem. I set my program up for 70 degree temperatures. At 50 degrees I add one click per 100 yards, and at 20 degrees I add two clicks per 100 yards. This keeps me right on.


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## Horsager (Aug 31, 2006)

It sounds like the only real curveball left is that the bullets B.C. may be a bit inflated.

Maybe that .508 is a "Metric" measurement of some sort!! :rollin:


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## Jiffy (Apr 22, 2005)

I'm pretty sure its magic.......


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## Plainsman (Jul 30, 2003)

Horsager said:


> It sounds like the only real curveball left is that the bullets B.C. may be a bit inflated.
> 
> Maybe that .508 is a "Metric" measurement of some sort!! :rollin:


That's about all I can think of. I am going to shoot five rounds over my chronograph at ten feet, then move it to 100 yards and shoot over it. I may have to order a new Chrony program. It will do ballistic coefficients for you by comparing velocities at different ranges. Of course I can just keep changing the B.C. until I create a ballistic chart that agrees with the real world. Cheap way out.

Jiffy, I may give up and say your right.


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## Plainsman (Jul 30, 2003)

Well, I have spent the last half hour playing with ballistic programs. I have used 800 yards as my indicator of trajectory shift. I have kept all but one variable constant in each scenario. The variables I tested included B.C., bullet weight,velocity, temperature, barometric pressure.
My thoughts before testing. Bullet weight should make no difference as long as B. C. remains constant. That held true. Temperature of ballistic should make no difference. That was also true. Barometric pressure, velocity and B.C. should make a difference, and I listed them below for a 155 gr bullet.

At 2900 fps and a BC of .425 minutes of holdover for 800 yards is 22 inches
BC of .450 = 22 inches , BC of .475 = 21 inches, BC of .508 = 20 inches, BC of .525 = 20 inches. None of these are enough to explain the ballistic change I am seeing.

Barometric pressure from 28.00 to 31.00 ranged from 19.6 to 20.7 minutes of angle holdover. Also not enough to explain anything.

Velocities:
2500 fps = 28 minutes of angle holdover
2600 fps= 26 minutes
2700 fps = 24 minutes
2800 fps = 22 minutes
2900 fps = 20 minutes
3000 fps = 19 minutes of angle holdover

I really need to chronograph in different temperatures again, and fire at least five rounds over the chronograph. Last time I fired only three rounds, and also I only let them cool for five minutes after removing them from my truck. I need to carry a box of shells in the back tool box so they are at ambient temperature then fire five rounds. Even though Varget is one of Hogdons extreme powders that is not supposed to be affected by temperatures as much as some powders I think I must be loosing 150 fps, or more, in velocity from 75 degrees down to 20 degrees. I can't remember, but my chronographed velocities I think were 2940 at 70 degrees and 2840 at 20 degrees. Velocity can be the only answer.


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## xdeano (Jan 14, 2005)

Good info Plainsman, I guess the scientist in you sure comes out when you can't find a reasonable answer. It gets to me too. I'll do some tests also if the weather gets better. But I think you have a few things narrowed down. I'll agree with the velocity conclusion.

Good stuff.

xdeano


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## Horsager (Aug 31, 2006)

If you're shooting East to West that's the same direction the earth spins. So 800yds might be further since you have to account for the earth's spin in that direction. Shooting from West to East would have the opposite effect. You have to shoot North-South to get a true measure.

Or, maybe just some more chronographing.


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## Jiffy (Apr 22, 2005)

:rollin: :rollin:


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