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I understand that rifles sighted at low elevations tend to shoot high at higher altitudes. I’ve read that this may be result of thinner air creating less drag but I’ve also read that this could be the result of less gravity forcing the bullet to drop. But I am not totally satisfied with these explanations.
 

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Density altitude affects the flight of the bullet, it is determined by correcting the altitude effects by air tempature.
 

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The idea of thinner air creating a bit less drag makes sense.

I moved from sea level to 4500 feet and have not noticed any outstanding difference in how my 17HMR shoots at 100 yards. If there is a difference, it is very slight.
 

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While less dense air at altitude does cause less drag and, therefore, less drop, you would have to make some serious altitude changes and some very accurate trajectory measurements over considerable distances to actually see the effect. For hunters, it is a far more theoretical than practical consideration.
 

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Density Altitude is what affects the atmospheric drag on the bullet and that's a combination of altitude and temperature. However, temperature is the bigger of the two.

Consider a 168grn .308 rd @2,629fps sighted in at 200yds, shooting with a scope 1.5" above the bore, shooting at 500yds (Calculations by Strelok):

On a 85 deg day and 100ft above sea level the drop at 500yds is 48.7". On a 35 deg day the drop is 55.67".

The same 85 deg day at 6,100ft above sea level the drop at 500yds is 47.95", only 3/4" less. At 35 deg, the drop is 55.0", only 2/3" less.

As you can see, temperature is by far a larger contributor than is altitude.
 

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Density Altitude is what affects the atmospheric drag on the bullet and that's a combination of altitude and temperature. However, temperature is the bigger of the two.

Consider a 168grn .308 rd @2,629fps sighted in at 200yds, shooting with a scope 1.5" above the bore, shooting at 500yds (Calculations by Strelok):

On a 85 deg day and 100ft above sea level the drop at 500yds is 48.7". On a 35 deg day the drop is 55.67".

The same 85 deg day at 6,100ft above sea level the drop at 500yds is 47.95", only 3/4" less. At 35 deg, the drop is 55.0", only 2/3" less.

As you can see, temperature is by far a larger contributor than is altitude.
Interesting :) Are the relationships linear? I'm not familiar with Strelok. Is his formula available on line? It appears that the change due to altitude and to temperature tend to offset each other as it is generally colder at higher altitudes. What are the numbers for 0 deg and for 12,100 ft comparison to these?
 

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I've heard of this. I think from sea level to say 12,000 feet you'd never notice the difference in your hunting rifle.

RJ
 

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well, from sea level to say 12,000 feet you'd never notice the difference in your hunting rifle.


That is true to a point, when ranges extend the difference is stark.
 

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For those of you who shoot with a scope and need some help with deciding where to hold, let me suggest a free app available at the Google Play Store for your android smart phone called Strelok (there may be one for an iphone but I don’t have one so I don’t know). You can load the specific information about your handloaded ammo (BC, weight, and velocity) for each caliber and bullet weight by specific rifle. If you use commercial ammo, you just select the brand and bullet weight. You also load in information about your specific scope including height, sight-in distance, and reticle you use (80 reticles listed in the free version).

Out in the field you simply input the range, wind, and degrees up or down you’re shooting and the app will calculate your trajectory and aim point giving you both tabular data and even better, an image of your exact reticle with exactly where you should hold it on to hit your target. I’ve used it to help us shoot 400-500yds at apples (my favorite long range reactive target) using my 68grn .223 handloads in both my long range AR with a 12x scope and my son’s Savage 10 with a Nikon Monarch 4-16 scope so far and it works pretty darn good. I’ve also used it to shoot at 300yds with a both my scoped Savage 99 in 300 Savage and my Winchester 88 in .308.

The phone’s memory holds both the program and data so you don’t need Wi-Fi or Cell connection to use the app out in the field. Best of all, you’ve already got your cell phone with you so this means you don’t need another piece of gear to lug around. There is also StrelokPlus for $5.45 that includes 145 more reticles if yours isn’t listed in the free version and he claims that more are added all the time.

You can check out Strelok and download the free version to your computer to try it out HERE. On the page he has a list of the reticles included in the free version and an additional list of those included in StrelokPlus. I don’t have anything to do with the app nor do I make any money off of mentioning it; I just found it, use it, and like it so I figure I’d mention it.
 

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when dealing with altitude the situation is not just temperature and altitude. there is a third factor that is directly related to altitude. Pressure. Gravity and air density affect pressure which affects the combustion inside your cartridge which affects the velocity of your cartridge at different elevations.
I grew up in Michigan. Elevations ranged from 2500 to 4500 feet generally. I moved to Wyoming back in "81" and have lived there since. My current home is at 6500 feet elevation. I hunt an area in the Snowy Range Mountains at a mean elevation of 8500 to 9500 feet. When I have people from Michigan Hunting with me I make sure they sight in at an elevation of at least 8000 feet if we're hunting my regular area. They are typically about 2 inches low when they go to sight in depending how they set their sights. Some people elevate their sights to compensate for longer distance shooting. Even when I sight in at home I have to compensate at least an inch for hunting at the higher elevations. I lived at 8600 feet elevation for ten years and when I went plains hunting deer and antelope I would have to resight the rifle. Cartridge pressure is an issue when reloading at High elevation and then going down to the plains and shooting. Especially when dealing with near full cartridge capacity powder charges. Temperature, Barometric pressure.,gravity all affect the cartridge.
My experience was that heavy loads created at lower elevation did not cause issues. Heavy loads created at higher elevations did make a difference. I reload cartridges from the 22 Hornet up to 338 Win in rifles. Mostly single shots now or bolts. because I handle most every case singly I see immediately the effects my reloads have on the cases.
I have a 30 caliber high capacity wildcat I developed that brought out the differences in Altitude very plainly. That is a bolt rifle that shoots a 200 grain 30 caliber bullet at 3100 feet per second. I was able to load about 3 more grains of powder at high elevation than I was able to use out on the plains. Temperature also plays an important role in that loads evaluation.
This is my personal experience and not derived from other sources. Formulas from other sources may say differently, but I wanted to interject some actual experience over 30 years of personal reloading and shooting. I had a personal 100 yard rifle range at 8600 foot elevation that I reloaded and shot at for ten years when I lived up in the Snowy range Mountains.
 

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I find this conversation interesting, but rather lacking. The guy that originally posed the question was interested in high altitude effects, I believe. I certainly wouldn't consider 9-10,000 feet as high altitude with regard to hunting--I doubt he did. In regard to hunting, I consider 12,000 feet plus as high altitude. I agree with what has been said in regard to the lower elevations, but when you get to high altitude, i.e. 13-16,000+ feet, I think you either haven't been there or haven't paid attention! It does effect bullet path! You will definitely see less bullet drop, and it doesn't require great distances to see it! At 13,500 feet, +/-, I noticed that my .284 Jamison (a 7mm wildcat that shoots basically the same as a 7 mm Rem Mag) with 140 gr bullets shot about 5" high compared to where it was sighted in at low altitude (4000 feet), and this at only 200 yards! At 15,000 feet, it was 8 inches! Obviously, enough for a missed shot if one hasn't compensated (or re-sighted). As one would suspect, temperatures were cool, but not excessive, about 15-20F. As one would expect, this effect is even more exaggerated if temperatures are cold. I've observed this several times--not a one time deal. I've learned to always re-sight in when hunting at high elevation, and if that's not an option, to compensate for it. As to answering the original question, I'm not sure if it was the result of lowered resistance due to the thinner air, or less gravity. Likely both (certainly both when being exact), but I suspicion that the reduced air density was the major contributor. Can someone please confirm or refute my suspicion? It has been something I have contemplated a lot, and discussed some. I haven't yet found a definitive answer, and I'm all ears! I think this is what the original poster was asking about, not the foothills.
 

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Shooting at High Elevation.

At high elevation the wind resistance is much less (at 12,000) feet compared to sea level. But if gravity is significantly different it would be very easy to measure simply by weighing on object at both elevations. On top of Mount Everest the air is 1/3 as dense at sea level, not sure about 12,000 feet
but been there scores of times and you can sure feel it.
 

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… Gravity and air density affect pressure which affects the combustion inside your cartridge which affects the velocity of your cartridge at different elevations.
You might want to rethink that. Using the ICAO International Standard atmosphere, going from sea level to 6000 feet changes pressure by about 2.9 psi, with altitude and standard temperature lapse with altitude both taken into account. Primers alone can unseat bullets, which, for a .308 bullet, typically takes about 60 lbs of force. At the .308 bullet's roughly 0.075² inches of cross-sectional area, it takes just over 800 psi of pressure to provide that 60 lbs of force. Priming mix typically weighs 0.5 grains and is metered to 0.01 grain precision in match primers, allowing ±2% variation. 800 psi ±2% is a spread of 32 psi. So the shot-to-shot pressure variation from the primer alone would be over ten times the change in atmospheric pressure you would get from that altitude increase under standard conditions. That ignores that, not counting squibs, the powder will actually be building pressure before the primer can overcome both bullet inertia and case neck grip to unseat the bullet. As a result, bullets don't actually start to move measurably until the pressure is several thousand psi, and shot to shot variation in that can be 10%, which swamps that 2.9 psi out of the picture by a factor of 100.

The difference in gravity at 6000 feet will average -0.057% from sea level. That's enough to turn a 50" drop at sea level into a 49.9715" drop at 6000 feet. A 0.0285" difference. Not much difference due to gravity change. And that's assuming you are shooting from an airplane over flat plains. If you get to 6000 feet by climbing a mountain, the mass of that rock may actually increase gravity slightly. World gravity maps show the increase in mountain ranges due to this. But it's small. Total variation over the Earths surface is less than 1%.

The bottom line is pretty much everything you see in bullet drop difference at high altitude is due to difference in air density changing drag so the bullet slows down more gradually. Losing less speed, the bullet's average velocity over a given distance is greater which decreases the transit time from gun to target. That gives gravity less time to act on the bullet to pull it down before it arrives at the point of impact.
 

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Elevation on Bullet path

Hi folks, Air does become less the higher you go but at 3000 ft when I have sighted in at sea level I don't see much difference. But at 6000 ft when I hunt Elk I do see a difference. Also don't confuse air density with Coriolis Effect when shooting East to West at over 500 yds but here is a program that will
help with how much difference is at what altitude.
gseven - Ballistic Program
its free but you do need to give a user name and password. they do not sell the info so its for tracking user information.
this is a good but simple explanation of the coriolis effect
https://www.youtube.com/watch?v=ouOYAlcvwOQ
https://www.youtube.com/watch?v=jX7dcl_ERNs
enjoy
 

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That's true, but Coriolis effect at 500 or 600 yards is about 1/10 the size of the effect of average air density difference between sea level and 6000 feet, and it affects your windage adjustment and not your elevation adjustment as air density change does. So its effect is on the wrong axis to cause that confusion.

Also, Coriolis effect is typically about 1/4-1/3 the magnitude of horizontal spin-induced deflection (aka, spin drift), which is why simple programs ignore it. For example, if I use longitude and latitude for Camp Perry, Ohio, shooting due North on Viale Range, and turn the Coriolis effect calculation off (this is in the QuickTARGET Unlimited 3DOF exterior ballistics program that comes with QuickLOAD), spin drift for a 175 grain MatchKing at 600 yards, fired from a 10 inch twist service rifle barrel at 2640 fps is 0.53 moa, or 3.3 inches. If I turn the Coriolis effect on, it moves the POI 0.13 moa to 0.66 moa total sighting error or 4.1 inches. So it's a hair over an eighth of an moa of effect at 600 yards, which is smaller than the best riflemen normally can hold in position shooting. If I start with a 12" twist, that cuts 0.14 moa, or 0.88 inches off the spin drift, making the spin drift to Coriolis effect correction about 3:1.

By 1000 yards, for that same load, Coriolis effect grows to almost 1/4 moa, and the best shots can just barely adjust for this. Mid Tompkins told us in a class that he felt sights with finer than 1/2 moa adjustment were not needed at 1000 yards. He learned to hold off 1/4 moa at that range, but he would never make a sight adjustment smaller than 1/2 moa because 1000 yard wind typically changed by more than 1/4 moa in the time it took to make the sight adjustment and get back on the gun.

Seeing how Coriolis effect increases with range, you can see how it is of much more significance to artillery. There you are often firing over a number of miles of range at high trajectories with lower average velocities and therefore much longer gun-to-target transit times that give the earth's rotation more time to move the target location.
 
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Unclenick, your explanation mirrors my own experience shooting to 1 K.
 

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All very interesting, but unless you are shooting at field mice, south to north in the northern hemisphere, at a range of miles and at an altitude where you will die in minutes without supplemental oxygen, none of it will make enough difference to worry about.
 
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