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Help: Gun vs.Target - steep shots, barometric adjustments with big elevation change?

 
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  #1  
Old 02-15-2008, 06:17 PM
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Help: Gun vs.Target - steep shots, barometric adjustments with big elevation change?

Gun vs target, long steep shots with big elevation difference.

Assuming that one day I get the 7mm AM that I’m saving for and decide to shoot a steep 2000 yard shot.

At 10491 ft elevation with a 200gr rbbt (.9 bc) going 3350ft/sec at the muzzle, according to JBM online ballistics calculator, gives a bullet drop at 2000 yards of 854 inches with a 100 yard zero. That’s if my target is also at 10491ft. (the same barometric pressure)

At 9306 ft elevation the drop is 870 inches over the same 2000 yard distance. Again, that’s if my target is also at 9306ft. (the same barometric pressure)

The difference being 16 inches ( 870”-854” = 16”) which seems like a fairly significant amount.

What if I shoot from the 10491ft station to the 9306ft station? 36 degrees downhill and 2000 yards away (verticle distance). How does the barometric pressure affect this drop? The air goes from thinner to thinker.

What if I shoot from the 9306ft station to the 10491 ft. station? 54 degree’s uphill. How does the barometric pressure affect this drop? Now the air goes from thicker to thinner.

Most ballistics programs I’ve looked at correct for Line Of Sight angle by taking into account the vertical vs. horizontal distance, and trajectory…but do they account for the difference in barometric pressure between the gun and target?

I’ve yet to see one that asks for pressure inputs at both gun and target. Are they adjusting for this using standard elevation pressures or are they ignoring it? I’m assuming they ignore it.

For shots at closer distances < 500 (sounds strange saying 500 yards is close), it probably wouldn’t make that much of a difference, but with the distances the 7mm AM is capable of shooting…. this has to come into play. How can you adjust for it?

Anyone have some insight on this? I can’t find any information on this anywhere.
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Old 02-15-2008, 07:19 PM
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There are many answers to give, but this guy and this answer are the most credible concerning drop corrections for angle.
Up/Downhill corrections

Lots of people reply but just pay attention to what Shawn has to say. If for some reason you don't understand it then send me (or Kirby) a PM

For a 7AM at the speeds you quote, the corrections I use are as follow:

Quote:
Effects @1000 yards
10 Degrees F = 0.1 inches of drop
1000 ft elevation = 0.2 inches of drop
You should put those numbers at the top of your paper drop chart
But as Shawn and Kirby and Goodgrouper and I and everybody who actually knows about these distances will advise you, if you have a PDA by all means use it. If you are pressed for time and feel you can get a good accurate shot off, the corrections for temperature and altitude I give you will do you OK but not great.

If you hang around this forum until this Fall you will see some spectacular shots made with a 7mm AM. I have pretty much run the little 240 Wby as far as it will go (1140 yards) and now it is time for Tiger Stripes to stretch it on out and cause tremors of terror through all of the antelope of Wyoming. I can hardly wait I am so excited. I have a new hunting partner and things are going to happen. I hope I get drawn on a good unit.

The 7mm AM is really a terrific cartridge and is great for bothering the people who think you need a 338 nuclear warhead to kill something.
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Old 02-15-2008, 07:53 PM
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I think he's specifically wondering how much the barometric pressure difference between the two altitudes (altitude of shooter vs altitude of target) will affect the shot (for really long shots at angles where the altitude difference seems significant).

I think he understands the drop difference induced by gravity/geometry, but he's specifically wondering how much difference just the barometric pressure difference between shooter location and target location (and the air in between) makes. This isn't something I've seen addressed anywhere, and something that none of the ballistics packages I'm aware of account for.

My gut feeling is that the effect on the actual effect ballistics will be relatively insignificant. I think to get some idea of the scale of the error the baro difference would induce, you could calculate the dope with ballistics software for a level shot at a standard baro pressure for 10,000 feet, then calculate the dope for the same shot with baro pressure that is standard at 9,000 feet or something. If the drop difference between the two at 1,000 yards or something is relatively small, then it's probably not worth worrying about. If it amounts to a quarter MOA or more, then it might be worth trying to compensate for. I would think that if you split the difference between the baro pressure at your current alt and the estimated pressure at the target's alt and plugged that pressure into your ballistics package, that the result would be a pretty decent approximation.
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Old 02-15-2008, 08:38 PM
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Quote:
Most ballistics programs I’ve looked at correct for Line Of Sight angle by taking into account the vertical vs. horizontal distance, and trajectory…but do they account for the difference in barometric pressure between the gun and target?
Matt, I suspect you are right about that he was asking. It was unclear to me if the fundamentals were understood. I think I gave the answer I felt he needed rather than what he asked.

OK, here is the best answer without installing a weather station where the elk is standing.

Shooting with a change of 1000 feet change in elevation from the drop chart is 0.2 inches of drop. Shooting with a change between the shooter's position and the animal's position (this is a 30 degree angle) is 0.1 inches of drop per 1000 yards due to altitude alone without considering temperature for the 7AM with a 200 grain Wildcat at 3360 fps (not 3350 fps). In other words for the 7mm AM you will be 1 inch high or low depending on whether you are shooting uphill or down hill as long as you make the correct cosine correction for the angle. This is less than one half of one click on a Nightforce or Luepold.
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Last edited by Buffalobob; 02-16-2008 at 08:24 AM. Reason: clarification and more clarification and even more
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Old 02-15-2008, 09:27 PM
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Quote:
Originally Posted by mattj View Post

I think he understands the drop difference induced by gravity/geometry, but he's specifically wondering how much difference just the barometric pressure difference between shooter location and target location (and the air in between) makes. This isn't something I've seen addressed anywhere, and something that none of the ballistics packages I'm aware of account for.
Thanks mattj... what you said is right..I'm not good with words...the difference at 2000 yards between the elevations given above is 16" which is .7639 minutes at that distance. (inverse tangent of (1.3333 ft divided by 6000 ft)) This seems significant to me.

Splitting the difference sounds like a good idea... but I'm guessing it's something that a physics person should be able to figure out exactly assuming constant variation of pressure as altitude changes. I'm sure it's already been done by someone.

If no one speaks up I suppose I could run some tests, make drop charts at the different elevations, then see what happens at the angles up and down, compare to values from existing ballistics programs, find out if there is a significant change, and if so, backwards match my results to a pressure matching my results...and THEN hopefully come up with a way to correct for the difference... That's a lot of variables and I just don't know if I'd be able to shoot good enough groups to get usable data.

Hopefully someone will know.
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Old 02-15-2008, 09:36 PM
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Or I could try this...

Quote:
Originally Posted by Buffalobob View Post
Shooting with a change of 1000 feet change in elevation from the drop chart is 0.2 inches of drop. Shooting with a change between the shooter's position and the animal's position (this is a 30 degree angle) is 0.1 inches of drop per 1000 yards for the 7AM with a 200 grain Wildcat at 3360 fps (not 3350 fps). In other words for the 7mm AM you will be 1 inch high or low depending on whether you are shooting uphill or down hill. This is less than one half of one click on a Nightforce or Luepold.
....sounds like it won't make a difference. Thanks Buffalobob
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Old 02-15-2008, 10:06 PM
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The adiabatic lapse rate is 5 degrees F per thousand feet.

If you are shooting up hill with a thousand feet elevation change then you have a temperature change affecting air density that causes a bullet to be 0.25 inches low at 1000 yards and you have an altitude effect on air density that causes the bullet to be 1.0 inch high at 1000 yards. The net affect is you will be 0. 75 inches high.

If you are shooting downhill with an altitude change of 1000 ft then the bullet will be .25 inches high due to the increase temperatures changing air density and it will be 1 inch low due to the altitude changing air density. and the result will be 0.75 inches low at 1000 yards.

If there is an inversion layer between you and the elk and the temperature is actually colder below the inversion then you will simply have to decide whether whether a 0.25 inch error in point of impact is important to you . The good news is that inversions occur when there is very little wind so you won't have to worry about the wind when shooting through an inversion.

The adjsutment factors I gave you in the first post were derived by running the ballistics repeatedly in JBM with changes in altitude while holding temperature constant and then running with constant altitude but different temperatures. They were developed for 1000 yards.

Now then if you are truly interested in 2000 yards then you should realize that the potential change in BC of the 200 grain Wildcat may be more of an unknown than the question you are asking. When you actually get you a rifle feel free to PM me and I will discuss my real results with you.
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