What effect does Altitude have on Bullet drop?

[Zerstoerer]

We zeroed in a rifle at 4000ft elevation.
(30.06, 180gr @ 2700fps)
Shot dead on at 100 yrds.

Weeks later the same gun prints 3-4 inches low
at the same distance at an elevation of 600 ft AMSL.

Could this be due to the difference in air pressure?
("Thicker" air at lower elevation, more resistance, less speed, more bullet drop ?)

What if we take this gun to a 9000ft elevation?
Will it "raise" the point of impact equally 3-4 inches above the 4000ft zero?

Ballistics - the never fully understood science...

 

[rangerruck]

actually once you start getting that high, the bullet drop exponentially falls less, it may not drop at all or even be high at 100.

 

[rwc]

Off hand, I would guess it has to do with the rifle's experience traveling between those two locations.

The air pressure differential between those two altitudes should be subsumed by gravity's strong hand. At least until the space shuttle starts allowing tourists with rifles...

 

[BobTheTomato]

Higher altitiude means thinner air. Thinner air means less wind resistance. Since drop is related to time after leaving the barrel, a bullet at a high altitude slows faster and will drop less per foot traveled. Hope that helps.

 

[strambo]

3-4" @100yds or 3-4 MOA sounds a bit much to be accounted for by only 3400ft elevation. Was it also colder that day at the lower elevation than when you were at 4000ft?

Dry air is also denser than humid air....so highest drag is at cold, dry and low altitude. Least drag at warm temp, high humidity and high altitude. These things don't all go together though, just categorizing the effects.

http://www.amazon.com/Understanding-Firearm-Ballistics-Robert-Rinker/dp/0964559846 Great book, the only section I had to read a couple times to understand was on the earth's rotation and its effects. The rest was easy to understand.

 

[dfaugh]

Just a WAG, without running any numbers, but I would bet that you'd see more variation shooting on a hot humid day vs. a cool less humid day (both at the same elevation) than you would with that amount of elevation change.

And in both cases, I'd bet that the difference would be so small that you would have difficulty measuring it. Definitely not 3-4 MOA.

 

[taliv]

the ballisticard range cards dont' start adjusting for altitude until 600yrds. they say (in the case of a 308win) a rough estimate is .5 MOA for each 2000' change at 600 yrds.

 

[Eightball]

Scientifically speaking, yes, elevation will affect bullet drop. The farther away from the earth's center you get, the less the pull of gravity will be, even if ever so slight. It's mathematically proven. Air resistance aside, gravity is different as well.

 

[c_yeager]

Less air density means the bullet will (probably) fly a little "straighter" (i.e. travel more horizontal distance for its verticle drop). It still falls at more or less the same rate, but it is making more horizontal progress as it slows down less in the thinner air.

 

[JohnBT]

Speaking of gravity and the decreasing pull as altitude increases, IIRC a jetliner at 35,000 feet weighs approximately 35 pounds less than it does at sea level. Somewhere in there is a ratio that would tell us approximately what a given bullet would weigh at 4,000 feet and how it would behave.

John

 

[T-Mac]

Drop will depend on FPS, or how far the projectile flies in a certain amount of time.

If thinner air at elevation results in more FPS down range, then less drop would be realized....no other factors involved.

.....................and, it does prove out, off the tee on any higher elevation golf course.

 

[mtang45]

If I remember my physics correctly, air density has little or nothing to do with bullet drop. Bullet drop is strictly a function of gravity. If it were possible to setup this demonstration, set a bullet on the edge of a table at exactly the same distance from the ground as the muzzle of your rifle and with the barrel perfectly horizontal (so not to create artificial lift). Drop the bullet off the table at exactly the same time as firing the rifle, both bullets would hit the ground at the same time.

The bullet in a straight drop off the table with no horizontal velocity will drop at exactly the same rate as the bullet exiting your barrel at 3000fps. Assuming the ground is perfectly horizontal both bullets would strike the ground at the same time.

Granted there is some change in gravitational pull between the elevations, but I think that would be countered by the DENSER air at the lower elevation which would tend to provide more lift/friction and thus a slower rate of fall.

Based on the above I believe a 3"-4" drop in POI is because the scope got banged in transit or some similar event.

I don't claim to be a physics expert so If I'm all wet in the above statements, I'm sure someone will hang me out to dry

 

[Matt-man]

If I remember my physics correctly, air density has little or nothing to do with bullet drop. Bullet drop is strictly a function of gravity.

There's a different issue at play here. What you say is true, but the amount of bullet drop at a given distance depends on how quickly the bullet gets there. If the bullet travels a given distance in less time, gravity has had less time to move the bullet downward and hence the bullet drops less. This is why high-velocity cartridges appear to be "flat-shooting."

 

[Blackfork]

Southern Hemisphere

Forget worrying about alititude! Start worrying about your bullet spinning the other direction when you get south of the equator! How about the phase of the moon? If it moves the ocean, don't you think it's going to be worth a couple of clicks on the back sight one way or another? And humidity, plus global warming! All the reloading data published before the invention of the SUV are off by a few data points.

Also: Cell phone towers affect the flight of the bullet!

 

[taliv]

heh, yeah, i heard the Army has a secret stash of M4 barrels with right-hand twists to refit all their rifles if we have to invade somethign in the southern hemisphere

 

[Blackfork]

Earth Velocity through space: resizing your continent.

I don't know about you but I think the fact that the Earth is hurtling through space at 176,000 mph, (at who knows what MPG), is affecting my standing scores in highpower. I'm nearly certain that the North American Plate lurches every now and then and I shoot an 8.

 

[Davo]

Air will be slightly less dense, but the combined effects of temp, elevation, humidity, and the rifle being moved around are likley to be the cause.

 

[Nickodemus]

I admit I did not read every post in this thread, and I am no expert, but I do remember this from Physics. I am sure many of you already know that gravity is a generally constant force of acceleration. I say generally constant because it is weaker as move away from the center of Earth, but in the few thousand feet range we live in, this change in gravity is insignificant.

Say you take 2 bullets that are at the same elevation with each other - If at the same instant you drop one bullet from your hand, and have the other one fired from a perfectly level rifle; both bullets will hit the ground at the same time. (Presuming the bullet doesn't hit anything in between and that the ground under your feet is flat, out to enough distance).

Gravity is a negative (that means towards earth) acceleration (that means velocity change over time) force of about -9.8 meters per second squared (Think it speeds up by the velocity of 9.8 meters per second, each second.) And in a vacuum a falling body would do that, consistently increase rate. However, air creates friction and that drag can curb the acceleration. As the velocity climbs so does the drag and eventually you hit the "terminal velocity" where the drag prevents further acceleration.

So what matters is how fast your bullet is moving forward, the push down from gravity is constant no matter where you are on the surface of Earth. The faster it goes forward the less it will drop for any given distance. This is only because the bullet has been on it's own in the air under the influence of gravity for less time at the given distance, if you increase its forward velocity.

So you are concerned with altitude which is all about the air pressure - Lower elevation has denser air and therefore higher drag to slow your forward velocity ("air speed" if you are familiar with aeronautical terms). Since your bullet has more drag in the denser air slowing its airspeed, by the time it gets to your target it has been in flight longer, and that additional time has allowed gravity to affect it longer. Since gravity is acceleration, this small additional time can have a very pronounced effect on the drop.

I would think shooting up or down wind would have a greater effect then elevation change, but have not tested that... at least you could calculate well for air pressure change. A barometer doesn't lie but the old windsock can really shift around on you.

Edit: I just read some posts... Unless you are shooting an artillery piece at over 100KM you do not have to worry about the Coriolis effect. - That opposite rotation for southern hemisphere thing.

 

[mtang45]

There's a different issue at play here. What you say is true, but the amount of bullet drop at a given distance depends on how quickly the bullet gets there. If the bullet travels a given distance in less time, gravity has had less time to move the bullet downward and hence the bullet drops less. This is why high-velocity cartridges appear to be "flat-shooting."

Exact-amundo, but I fail to see how that relates to the same rifle/caliber/bullet/weight/velocity and range having an altered POI (by 4 inches!) due to elevation (air density).

My comments above were to the folks saying that POI can be effected by air density by as much as 4" at 100yds. My vote still goes to banged scope or mount.

 

[Ol` Joe]

According to RCBS load there is ~an inch difference in drop at 500 yds from a 30-06 with a 165gr Nosler Partition @ 2800 fps with a altitude variation of 6000ft. Note this is at 500 YARDS not 100!
The lesser air density does affect the bullets travel the same as the BC of the bullet affects it. The drop is a result of time under gravity and variation in air density will cause the bullet to slow down at different rates altering the time of flight to the target.

 

[Myself]

What effect does Altitude have on Bullet drop?

The higher up you are the further the bullet can drop.

 

[Matt-man]

Exact-amundo, but I fail to see how that relates to the same rifle/caliber/bullet/weight/velocity and range having an altered POI (by 4 inches!) due to elevation (air density).

Lower air density = less velocity loss = less bullet drop. It's not "little to no" effect when even freeware ballistic calculators model the difference in drop due to altitude.

You do have a point in saying that a 4MOA POI shift is not likely to be due to altitude changes, but stating that bullet drop is completely a function of gravity is erroneous. If that was true we wouldn't care about ballistic coefficients, would we?

Edit: Clarification: Bullet drop, observed alone, is indeed a function of gravity. However, when comparing the paths of two projectiles flying through air, gravity is not the only factor in play. Time-of-flight plays a major factor here as Nickodemus has pointed out.

 

[mtang45]

Lower air density = less velocity loss = less bullet drop. It's not "little to no" effect when even freeware ballistic calculators model the difference in drop due to altitude.

You do have a point in saying that a 4MOA POI shift is not likely to be due to altitude changes, but stating that bullet drop is completely a function of gravity is erroneous. If that was true we wouldn't care about ballistic coefficients, would we?

Edit: Clarification: Bullet drop, observed alone, is indeed a function of gravity. However, when comparing the paths of two projectiles flying through air, gravity is not the only factor in play. Time-of-flight plays a major factor here as Nickodemus has pointed out.

I think we are all talking the same thing but I just want to clarify some terms. Several folks are talking about time of flight. Time of flight for all projectiles fired from the the same height above the ground (with a perfectly horizontal barrel) IS THE SAME. Ballistic coefficients refer to the ability of a projectile to maintain a velocity. Thus a higher ballistic coefficient projectile will travel further in the same amount of time than a projectile with a lower ballistic coefficient, but the amount of drop, in equal time increments, is identical.

A 40gr .22 LR fired at 1100fps will hit the ground at exactly the same time as the 55gr .223 fired at 3000fps. Certainly not in the same location, but at the same time. At one half second in the flight path the .22 LR would be 183 yards down range (not accounting for deceleration) and the .223 would be 500 yards down range BOTH will have dropped approximately 50 inches.

Higher ballistic coefficients simply get you further down range in the same amount of time. Rate of drop does not change.

According to RCBS load there is ~an inch difference in drop at 500 yds from a 30-06 with a 165gr Nosler Partition @ 2800 fps with a altitude variation of 6000ft. Note this is at 500 YARDS not 100!

I would say given a 6000 ft difference in elevation a change in drop of 1 inch at 500 yards is "little to no" effect.

 

[svtruth]

The Elevator of Truth

needs to try this out. Both gravity and air density will be less at altitude.