Jump to content
Brian Enos's Forums... Maku mozo!

Altitude


Recommended Posts

Does anyone have any hard data on the effects of altitude on pistol ammo, i.e. chronographing at sea level versus doing it at 2000 or 3000 feet?

I think that conventional wisdom would say that the thinner air at altitude would impart less drag on the bullet, resulting in less velocity drop at extended ranges, but I don't think that drag would be an issue at 10 feet downrange where the chrono is....

Thanks!

Link to comment
Share on other sites

I haven't done altitude testing myself but I do know that if the load is 130PF at 3000 feet, you may/will have problems at sea level.

Also, humidity will be a factor too. I was chronoman at a major match last year and DQ'd a person for not making PF. He declared 165 and made it 2 weeks earlier - less than 50% humidity - but in 95% humidity he made 162. Same altitude.

Let's not make this a DQ or fun factor.

Edited by RePete
Link to comment
Share on other sites

I haven't done altitude testing myself but I do know that if the load is 130PF at 3000 feet, you may/will have problems at sea level.

Also, humidity will be a factor too. I was chronoman at a major match last year and DQ'd a person for not making PF. He declared 165 and made it 2 weeks earlier - less than 50% humidity - but in 95% humidity he made 162. Same altitude.

Let's not make this a DQ or fun factor.

please............! you can not DQ someone for not making power factor, they just shoot for fun.

lynn

p.s. altitude, humidity have little or no effect on chronographing. this has been researched. temperature will have an effect on some powders.

Link to comment
Share on other sites

I haven't done altitude testing myself but I do know that if the load is 130PF at 3000 feet, you may/will have problems at sea level.

Also, humidity will be a factor too. I was chronoman at a major match last year and DQ'd a person for not making PF. He declared 165 and made it 2 weeks earlier - less than 50% humidity - but in 95% humidity he made 162. Same altitude.

Let's not make this a DQ or fun factor.

please............! you can not DQ someone for not making power factor, they just shoot for fun.

lynn

p.s. altitude, humidity have little or no effect on chronographing. this has been researched. temperature will have an effect on some powders.

Hate to tell you, but it does. The conditions I listed above where the same temperatures.

When the scores are not posted, that's a DQ. And I said lets NOT make this aDQ/fun spin off.

Link to comment
Share on other sites

I haven't done altitude testing myself but I do know that if the load is 130PF at 3000 feet, you may/will have problems at sea level.

Also, humidity will be a factor too. I was chronoman at a major match last year and DQ'd a person for not making PF. He declared 165 and made it 2 weeks earlier - less than 50% humidity - but in 95% humidity he made 162. Same altitude.

Let's not make this a DQ or fun factor.

please............! you can not DQ someone for not making power factor, they just shoot for fun.

lynn

p.s. altitude, humidity have little or no effect on chronographing. this has been researched. temperature will have an effect on some powders.

Hate to tell you, but it does. The conditions I listed above where the same temperatures.

When the scores are not posted, that's a DQ. And I said lets NOT make this aDQ/fun spin off.

Sorry, but a DQ is just plain wrong for going sub-minor. Appendix C2 USPSA Rulebook.

If the resultant power factor fails to meet the minimum power factor floor for the relevant Division, the competitor may continue shooting the match, but not for score or match recognition.

If a DQ was issued, you don't get to shoot anymore.

Link to comment
Share on other sites

well if this helps, at coors field in Colorado where balls seem to carry out of the ballpark alot, they put the balls for the games in a humidor. so if I had to make an educated guess, I would think the bullets would seem to fly faster at higher altitudes. JMHO

Link to comment
Share on other sites

Does anyone have any hard data on the effects of altitude on pistol ammo, i.e. chronographing at sea level versus doing it at 2000 or 3000 feet?

There are several things that effect velocity in a rifle when you increase your altitude and these will make a difference for long distance shooters. But for pistol, I don't think that altitude, per se, has that much effect. However, barometric pressure, humidity, and particularly temperature will have some effect - particularly since some powders are more sensitive to temperature changes than others are.

If there are any hard and fast rules, I have not heard of them - but there's a whole heap of stuff I've never heard of.

Link to comment
Share on other sites

I haven't done altitude testing myself but I do know that if the load is 130PF at 3000 feet, you may/will have problems at sea level.

Also, humidity will be a factor too. I was chronoman at a major match last year and DQ'd a person for not making PF. He declared 165 and made it 2 weeks earlier - less than 50% humidity - but in 95% humidity he made 162. Same altitude.

Let's not make this a DQ or fun factor.

please............! you can not DQ someone for not making power factor, they just shoot for fun.

lynn

p.s. altitude, humidity have little or no effect on chronographing. this has been researched. temperature will have an effect on some powders.

Hate to tell you, but it does. The conditions I listed above where the same temperatures.

When the scores are not posted, that's a DQ. And I said lets NOT make this aDQ/fun spin off.

Sorry, but a DQ is just plain wrong for going sub-minor. Appendix C2 USPSA Rulebook.

If the resultant power factor fails to meet the minimum power factor floor for the relevant Division, the competitor may continue shooting the match, but not for score or match recognition.

If a DQ was issued, you don't get to shoot anymore.

Lynn:

The topic is altitude and chrono velocity, lets keep it there.

I never said what game, you are just assuming USPSA.

Remember what happens when you assume.

Link to comment
Share on other sites

I never said what game, you are just assuming USPSA.

Remember what happens when you assume.

OK What game are you talking about then.?

AND if you come to a predominantly USPSA forum without mentioning another game one is bound to assume you are talking USPSA.

Just sayin'

Link to comment
Share on other sites

I never said what game, you are just assuming USPSA.

Remember what happens when you assume.

OK What game are you talking about then.?

AND if you come to a predominantly USPSA forum without mentioning another game one is bound to assume you are talking USPSA.

Just sayin'

Point take Kevin.

IDPA.

Link to comment
Share on other sites

do you really think that the 10-15 feet from the muzzle to the chrono, the velocity has a chance to change?

THAT'S what I was thinking. The long-range effects of thinner air are well-documented, but I was checking to see if short-range (i.e., USPSA chrono distance) would be affected by a thousand feet or two thinner air.

Thanks for the replies.

Link to comment
Share on other sites

do you really think that the 10-15 feet from the muzzle to the chrono, the velocity has a chance to change?

Depends. Like I said, it's not just the altitude, it's all the other stuff.

And, again, I don't know how pistols are effected but I can tell you that when I go from sea level to 3000', the chronograph readings on my rifles (.223 and .308) will drop about 50 fps.

Link to comment
Share on other sites

  • 3 weeks later...

well if this helps, at coors field in Colorado where balls seem to carry out of the ballpark alot, they put the balls for the games in a humidor. so if I had to make an educated guess, I would think the bullets would seem to fly faster at higher altitudes. JMHO

They put the baseballs in a humidor because in Denver the air is so dry the balls dry out, become harder, then fly further due to being dry/hard.

Last time I chrono'd at a lower altitude with higher humidity, my power factor went up. Your results may vary.

Link to comment
Share on other sites

do you really think that the 10-15 feet from the muzzle to the chrono, the velocity has a chance to change?

Depends. Like I said, it's not just the altitude, it's all the other stuff.

And, again, I don't know how pistols are effected but I can tell you that when I go from sea level to 3000', the chronograph readings on my rifles (.223 and .308) will drop about 50 fps.

It's temperature, altitude, humidity, barometric pressure, pressure density, and a myriad of other things.

"for the most part", those things are inter-related effects on barometric pressure - temperature affects baro pressure, humidity affects baro pressure, altitude affects baro pressure, etc. So, as far as the bullet's flight is concerned, barometric pressure is sort of a key variable, with temp and altitude being inputs. You can get good [rifle] ballistic info just using "absolute pressure" (actual baro pressure measured at your actual location), and can largely ignore temperature and altitude.

All of those things, though, result in effects on the trajectory of the bullet along its ballistic path. Higher altitude means less air-density, which means less resistance, which means the bullet retains velocity longer, which means it drops less.

*None* of those things make enough difference in the 10 feet between the barrel and the chrono to produce 50fps difference in chrono readings. at that distance, you're still primarily dealing with internal ballistics, not external. In other words, at that distance you're still dealing with how the powder moved the bullet down the barrel, not how the bullet will interact with the air along its trajectory.

What generally affects chrono readings is temperature's effect on two things: the powder, and the barrel. Many powders are temperature sensitive (one direction or another). A powder which generates 1000fps at the muzzle in 70F temperature can produce drastically different velocities if the ammo is left in an ice-chest, or left out to bake in the sun. And, that ammo that generates 1000fps out of a cold barrel can easily produce significantly different velocities out of a hot barrel (plus the effect on powder if the ammo is chambered in the hot barrel long enough to heat up).

If your rifle chronos significantly different at sea level than it did at 3000 feet, it's almost entirely because of temp, not altitude or humidity.

Edited by jakers
Link to comment
Share on other sites

Distance from the chrono is irrelevant. The relevance is barometric pressure and altitude. Check the Siera manual. It has the formula for speed loss or gain due to change in pressure / altitude.

As previously stated the actual altitude is not the be all and end all. With a change in altitude you have changes in temp, humidity and pressure. All these do affect velocity and with a big enough change in one or all of the variables there could be a drop off as much as 5 or more in pf. Hence the reason to ensure you are 4 or 5pf over and maybe a little more.

The projectile will reduce in speed by as little as 2fps per foot of travel and as much as 7fps per foot of travel. Again subject to variables. This information was got from Sierras testing department. Instrumental velocity is adjusted to give actual muzzle velocity at most testing facilities.

Luckily both IDPA and IPSC / USPSA state that the chronograph must be at X feet from the muzzle. NRA AP does not, their rules state muzzle velocity and they place the chrono at sometimes 10feet. This however does not give muzzle velocity but velocity at chronograph.

Oehler would be good folk to get data from.

Link to comment
Share on other sites

With respect, this:

Distance from the chrono is irrelevant.

conflicts with this:

The projectile will reduce in speed by as little as 2fps per foot of travel and as much as 7fps per foot of travel.

Distance to the chrono is important insofar as it is necessary to be consistent. Testing one day with the chrono at 10 feet, and another day - identical conditions but with the chrono at 30 feet - there's no question you'll get slower readings on the second day.

And, yes, you're correct that bullets lose more or less speed (they never gain speed ;-) ) differently at different altitudes. But that phenomenon really isn't about altitude itself, it's about altitude's effect on the air that the bullet is going through.

Put it this way - if I chrono at sea-level on a day when my Kestrel indicates an absolute [local] barometric pressure of 29.54 in-Hg, and I chrono a different day at 3000 feet of altitude, in different temp and humidity, but the absolute barometric pressure is the same same 29.54 in-Hg... I expect that my round will perform the same on both days

-- same chrono readings (assuming the ammo hasn't been cooled or heated)

-- same ballistic trajectory

-- same velocities along that trajectory

This is because the "absolute pressure" - the barometric pressure measured right here, right now, without adjusting it for altitude - INCLUDES the effects of temperature, humidity and altitude, and is a relatively complete indicator of how much effect the "air" is going to have on the bullet.

Put differently - I don't have to know my altitude, either when I chrono or when I shoot. All I have to know is the absolute pressure when I chrono'd, and the absolute pressure where I am now. With those two things, I have everything I need to calculate my dope. In fact, in the JBM engine (and others), if you indicate "absolute pressure" as an input, it disables input of temp, humidity and altitude - giving it the absolute pressure is all the info it needs.

Link to comment
Share on other sites

Air pressure, altitude and humidity are irrelevant for IPSC chrono use.

A bad BC .45 bullet would only lose about 1 fps per foot of travel at the bottom of death valley. At the top of Mt Everest, it would lose something like 0.9 fps/foot for the first twenty feet. Chronos vary far more than that, even the two-chronos-in-one Oehler 35's.

Link to comment
Share on other sites

  • 2 weeks later...

Taking things out of context and without reference to the question asked is not particularly bright. Especially leaving ou tthe rest of the paragraph, not once but twice.

I answered the question he asked. He managed to only read some of the words.

Maybe you should look for employment in the news industry. Unfotunately Rupert shut down your best likely employer.

Link to comment
Share on other sites

If those comments are directed at me, I meant no offense.

I was only trying to make two points, in response to the OP's question (and the rest of the thread):

1) Altitude is not an "independent variable". It is *one* of a set of inputs that affect the bullet's flight. Put simply, the main purpose of using altitude is to "adjust" the model to reflect the density of the air the bullet is traveling through. It has almost no meaning by itself

-- a bullet fired at sea-level, in atmosphere at 30.2-inHG "absolute" (unadjusted) pressure, will behave *exactly* the same as a bullet at the same velocity at 6000 feet, in 30.2-inHg "absolute" pressure. The altitude's value is in "adjusting" the reported "station pressure" to model local conditions. Just noting change in altitude is meaningless, you have to consider both altitude *and* pressure.

2) distance to the chronograph *is* an independent variable. Change the distance to the chrono, you'll get different results. A bullet fired over a chrono 10 feet away will return a faster "measured velocity" than the same bullet fired over a chrono 30 feet away.

Link to comment
Share on other sites

Update:

Although this is far from a scientifically-defensible result, my ammo that is loaded and chrono'ed in Atlanta (1000' elevation)ran about 40 FPS faster in Vegas (3311') for the Production Nationals.

4.1 VV N-320 behind a 124 MG JHP in a KKM'ed Glock 34.

Not that it's an imperical data; just an observation that alleved my fears a bit.

Their scale to weigh the guns is another matter altogether.... :rolleyes:

Edited by Braxton1
Link to comment
Share on other sites

Update:

Although this is far from a scientifically-defensible result, my ammo that is loaded and chrono'ed in Atlanta (1000' elevation)ran about 40 FPS faster in Vegas (3311') for the Production Nationals.

4.1 VV N-320 behind a 124 MG JHP in a KKM'ed Glock 34.

Not that it's an imperical data; just an observation that alleved my fears a bit.

Their scale to weigh the guns is another matter altogether.... :rolleyes:

Did you happen to note humidity and temperature?

Link to comment
Share on other sites

Shred's comments seem to be on the money to me.

There are too many variables: too many things like ambient temperature and altitude and distance to the sensors that you can't control, and too much variability in chrono measurements to make it that useful to worry about some small difference that you might only be able to pick up under carefully controlled laboratory conditions, only to have it get swamped in the much larger random ups and downs caused by everything together.

Better to load to 5 PF over the threshold (10 would be better), and be done with it.

Just my two cents...

Link to comment
Share on other sites

Better to load to 5 PF over the threshold (10 would be better), and be done with it.

Just my two cents...

I agree 100%. I'd just never been to a Vegas Nationals and been handed THAT much of an environmental change.

In Atlanta in the summer, we're dealing with 1000' elevation, 95 degrees, and 80% humidity. That is a lot to change. It isn't like going to Barry, where the conditions are almost exactly the same.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...