On another forum a guy and I bantered about the effect elevation had on vel/pressure. He felt that because he lived at 8000 ft he could probably squeaze another 50fps out of a max load. I tried to tell him that it would not have any positive effect...only down range. He replied that he read that Lazzeroni did load development at high altitude to help achieve the advertised velocities. Any opinions on this??

Opinion and this is opinion only............

As for the pressure differnces between the case and the outside air, there would only be the gage pressure acting as a driving force for the bullet, as the absolute pressure would cancel out whether at sea level or 8000 ft.

A difference might be seen if the work required to push the less dense (and therefore less mass) air out of the barrel at 8000 ft. is measureably less compared to the greater mass if air in the barrel at sea level.

It would seem that this would be insignificant but I don't know. And I ain't doing any calcs to prove it one way or another!

Will,
Std pressure sea level 14.7 psia
Std pressure at 10,000 ft 10.1 psia

Assume load at sea level yields 58000 psig
load at 10,000 feet would yield 58,004.6 psig
I then entered the data posted by Collins 200Grn Nosler Partition Alliant Reloader22 into a Statistical Program (Minitab) and generated a regression plot.


Based on the Regression Equation I calculated the velocity at 58000 PSI
V = 1893 + 0.01298 X 58000 = 1893 + 752.84 = 2645.84 fps
and at 58004.6
V = 1893 + 0.01298 X 58004.6 = 1893 + 752.899 = 2645.90 fps
essentially equal.

So the muzzle velocity is unchanged at altitude. Its true that the density of the air is less at altitude and that will effect the drop at target since the drag is less.

Pat

I don't disagree but assuming the cartridge case is leak proof the gage pressure inside the case will be the same, as it is only the absolute pressures that are different.

P (gage) = 58000

P (abs at SL) = 58000 + 14.696 {3 decimals just because I remember them!]

P (abs at 8K ft.) = 58000 + 10.4 [whatever you said]

But I don't think the cartridge sees absolute pressure, just gage pressure. I guess!

If you had a vacuum pump on the end of the barrel, will not your pressure gage still read 58000 psi? I think it will.

The decreased pressure due to the vacuum pump would help the bullet along its way due to decreased resistance by the air, or lack of, right? Maybe?

I guess one could calculate the work required to move the 2 feet of air in the barrel in whatever fraction of a second it takes, for the two different weights (masses) of air and subtract that from the bullet energy at SL and see if that little amount would change the bullet velocity. It will change it but I'm guessing that it is very, very tiny!

I'll shall wait for your calculations.

Will,



A) sealevel condition
B) 10000 ft condition
C) 10000 ft condition

You are correct that it the delta pressure that moves the bullet, example C has the same delta pressure as B) and is valid to compare to A) the sealevel condition. Therefore the comparsion in my prior post accurately answers the question (does altitude affect muzzle velocity?)

Your point that the effort to push the air infront of the bullet is effected is valid. The difference between the work performed will be directly proportional to the air density differences. Air Density decreases at a rate of 2.9% - 3.0% for each 1000 ft. of elevation above Sea Level.
The density of air at sea level is 1 lb. For 13 cubic feet. Assume .30 inch bore and 24†barrel volume is 3.414 X .30 x .30 /4 = 0.07068 cubic inches.
0.07068 cubic inches x 4.451e-5 = .00000314 lbs of air to move at sea level
bullet + air = 200gr/7000gr/lbs=0.02857 +.00000314 = 0.0285745 X 7000 =200.03738 grains total

at 10,000 ft the total weight to move is 200.037379 grains (bullet + air)

If I used proper precision there would not be a difference.
The change in air mass is well below the variation in the bullets.

So I still say no effect on muzzle velocity.
Pat

quote:

Originally posted by pdpdad:

at 10,000 ft the total weight to move is 200.037379 grains (bullet + air)

That's a better way of looking at it than the way I did. Just a heavier bullet! Thanks.

Yep, its the difference in the weight (mass) of air in the barrel as a percentage of the mass of the total ejecta (bullet + powder + air in barrel - mass converted to energy) that will be proportional to the reduction in pressure. In other words, nothing.

But mess with your friend's small mind by telling him that the big reduction in pressure is offset by the lack of pressure on the outside of the action, pressure which otherwise helps the action resist the internal pressure of the gas. In other words, tell him that the action is effectively weaker at high altitude. This will have him crosseyed for weeks and he'll probably be afraid to hunt elk above the timberline for fear of his action letting go due to low air pressure.

/

quote:

Originally posted by kraky:
On another forum a guy and I bantered about the effect elevation had on vel/pressure. He felt that because he lived at 8000 ft he could probably squeaze another 50fps out of a max load. I tried to tell him that it would not have any positive effect...only down range.

it would seem both of you have way too little to do!

quote:

So at altitude barometric pressure goes down so does air density and drag decreases, thus velocity would increase for the same load and bullet parameter, midrange trajectory is up and drop is less when compared to sealevel

Alf all this is true, but it relates to the bullet at a point foreward of the muzzle, not flush at it. The drag on a bullet at elevation will lessen and allow for more velocity to be retained but it does not add an measurable amout to the velocity at the muzzle as I think is the claim in the question.

quote:

Originally posted by vapodog:
it would seem both of you have way too little to do!

Ahhh... but you are forgetting the important part... because of the less dense air at altitude that mounting rifle ou are lugging around is 1/7,000,000,000 lb lighter!

Gravity is less as the distance from the center of the mass is increased.

Throw that into your calculations.

quote:

Originally posted by Allan DeGroot:
Ahhh... but you are forgetting the important part... because of the less dense air at altitude that mounting rifle ou are lugging around is 1/7,000,000,000 lb lighter!

Hate to pop your bubble, Allan, but the denser the air the more an object tends to "float". Because your rifle is less boyant at 10,000 feet, it will be heavier. That mountain rifle is likely to feel like an unlimited bench gun if you get it way up there on top of the mountain !

Now, if you use it to shoot fish underwater, it will be as light as a feather. Of course, replacing the air in the barrel with water brings its own set of issues to deal with.