Chamber pressure changes with altitude

[BallisticsGuy]

Yeah. Strain gauges aren't hard to set up and I already wrote the code to datalog on an arduino for a previous project.

 

[bigngreen]

Without being able to control atmospheric pressure alone like we can temp there is no way to conclusively test this, I've seen a lot of small and really insignificant things be taken into account but never have I heard of atmospheric pressure with chamber pressure be put together! I will typically change 4-5000 ft in elevation through a day of hunting and even at stupid far distances I've seen not indication of something changing other than the variables already under control.

 

[BallisticsGuy]

Controlling atmospheric pressure isn't necessary really. It only has to be known to quantify MV and pressure departures from known sea level data. What I'll end up doing is going to my normal range which is usually around 29.3inHg, do a run of tests on cases with roughly 25, 50 and 90 grains capacity (so roughly .223, .308 and belted magnum) then run up to a little spot I know at around 9k and then circle back down the hill to another spot at 6k and the last at 3k. With MV's, 100m zero shift data and pressure data we'll get enough data points that we should be able to determine what's happening or at least form a hypothesis that can be further tested against.

 

[Michael Eichele]

Controlling atmospheric pressure isn't necessary really. It only has to be known to quantify MV and pressure departures from known sea level data. What I'll end up doing is going to my normal range which is usually around 29.3inHg, do a run of tests on cases with roughly 25, 50 and 90 grains capacity (so roughly .223, .308 and belted magnum) then run up to a little spot I know at around 9k and then circle back down the hill to another spot at 6k and the last at 3k. With MV's, 100m zero shift data and pressure data we'll get enough data points that we should be able to determine what's happening or at least form a hypothesis that can be further tested against.

Keep us posted. Let us know your load specs too. In my experiences, it's been loads with poor densities that have yielded higher velocities at higher altitudes. For example: my 300 win mag loaded with 71.5 grains of H1000. Clearly under filled. The other, my 6.5-284 running 51.0 grains 4831SC. Again, a very under filled case. My high load density 308 loads never seem to suffer.

 

[Pdvdh]

One of you doesn't seem to want to arrive at an answer, happy to assume someone else had that on their list of things to do in the last century but provides no study abstract to support the assertion. The other already has their answer but its derivation from a tiny sample doesn't lend it to being applied generally so the former calls it an anecdote instead of what it is which is empirical experimentally derived evidence deserving of further investigation. It's deserving of investigation simply because the result was not null. The fact that data may not have been perfectly scientifically gathered only promotes the case for further testing so that the result can be verified or refuted.

A final thought, accusations of logical fallacy use are like accusations of misspellings and poor grammar. They're almost always accompanied by, if not surrounded by, the very thing they decry. This thread was no different.

An initial thought, speaking on behalf of others is worse than accusations of misspellings and poor grammar. It's almost always accompanied by arrogance and efforts to mislead, if not surrounded by, the thing they decry. Your post was no different. Why would any person pretend to know the mind and intentions of another, and then, even worse, express their self-endowed gift of understanding on a public forum.

Secondly, I arrived at an answer, and it's a stretch beyond all imagination for you to have read my prior posts, and not recognized it. The fact that I stated no connection between chamber pressure/MV and altitude has been demonstrated over generations of empirical testing and understanding of small arms fire was to set the stage for the most "logical" conclusion of those hard facts. It doesn't negate or diminish my expressed position/"answer" that any connection between altitude, in itself, and MV/chamber pressure, is nothing more than a myth. In the same camp of myth that some rifles magically improve precision with increasing range.

Thirdly, you have a most liberal definition of empirical experimentally derived evidence. If the information posted in this Thread supporting the contention that altitude affects MV meets your definition of empirical, experimental evidence, then you might consider having someone else review the design and implementation of your empirical evidence collection process, before implementation. It only qualifies as empirical evidence, to most scientists, if the experiment and data collection was designed and implemented using means, methods, and credible equipment that ensures any other qualified individual(s) can repeat the data collection process, with statistical relevance. This means control over all other possibly affecting influences, particularly when the cause and effect being studied hasn't been substantial enough to have become apparent to the extent of having been further studied and verified over generations of previous small arms fire study and understanding.

Do you have any sound science-based hypotheses to suspect that altitude alone effects changes in MV from small arms? I can't think of one. Back to the rifles that were long respected as capable of improved precision at greater ranges - a person would have to subscribe to the belief that the bullets have an inherent self-guidance ability that constantly and consistently corrects normal bullet dispersion in only one scientifically inconceivable manner - towards improved precision. Many over the years also mistakenly considered the apparent abundance of anecdotal evidence to rise to the standard of empirical evidence, thereby supporting and perpetuating the myth.

 

[Pdvdh]

Keep us posted. Let us know your load specs too. In my experiences, it's been loads with poor densities that have yielded higher velocities at higher altitudes. For example: my 300 win mag loaded with 71.5 grains of H1000. Clearly under filled. The other, my 6.5-284 running 51.0 grains 4831SC. Again, a very under filled case. My high load density 308 loads never seem to suffer.

The additional emphasis concerning powder load density just undermined the notion of the existence of empirical scientific collection of evidence supporting the myth that altitude, in and of itself, affects MV. We now have another possible factor - powder load density of the cartridges. Which implies even another possible cause - specific brands, specific powder identity within brands, and even lot numbers within the identical powder selected and used.

If altitude, in and of itself, affected MV, wouldn't one expect to measure that affect on MV with cartridges regardless of powder load densities?

The observation being observed and expressed has now turned to a suspicion that different powder load densities combined with differing altitude, affects MV. Which would suggest unstable powder load density much more than altitude as the cause and affect.

With a hop, skip, and a jump, the possible combination of contributing factors approach infinity, and become indistinguishable from one another. Unless altitude alone has an overwhelming affect on MV, which causes MV to rise clearly above all the extraneous combinations of possible causes that combine to create a lot of meaningless background noise and variation in MV, the empirical data collection process becomes an effort in futility.

I'm unaware of any scientifically sound hypotheses suggesting how altitude could affect MV, and I can't think of a single one on my own, but I'm all for the effort provided it's on others' dime and time. If a person can concoct the perfect mixture of a large enough number of variables and influencing factors, we know we can cure cancer. Which is the inverse of stating, if the empirical scientific collection of data isn't carefully designed and controlled, the data measured and collected will be meaningless.

 

[Mikecr]

If actual MV change with altitude has been observed, then why would that be?
I'm not intelligent enough to come up with a credible hypothesis..
Inches of mercury pressure change in the case, or bore, seems too small to affect internal ballistics.

External ballistics is another matter, but different all together.

 

[Pdvdh]

If actual MV change with altitude has been observed, then why would that be?

Well it's been a good long time. No emperical evidence. The answer to the question why would that be, is there is no why, because there is no that.

 

[RockyMtnMT]

We have not noticed changes in velocity at different altitude. Have suspected it, so have not checked it. We have noticed that loads developed at our altitude of 3000' for top end performance will over pressure at 800' elevation. My conclusion to this is the increase in atmospheric pressure at lower elevation adds to the pressure of the load causing the over pressure. This would indicate a load developed at low elevation would have less pressure at higher elevation. Now in my simple mind pressure is an indication of vel. More pressure = more vel. This would have the opposite effect of the original post. Right? When this over pressure happened to us we fixed it by reducing the load 2 or 3g. I don't remember exactly. And I do not remember if the vel wound up being lower or not. Brian may have that data. For our situation we simply do not work up top loads that will be used at lower elevation anymore.

I am not even sure if this is relative to this conversation or not.

Steve

 

[Pdvdh]

Steve,
The change in pressure over 3,000' El is about 3psi.

The chamber pressure in the cartridge case is around 60,000psi. A 3psi increase or decrease to 60,000psi amounts to a 0.005% change in chamber pressure. Not discernable / measurable.

Not the source of sensed increase or decrease in chamber pressure. Or increased / decreased MV.

I don't think there is a science-based explanation for your observation. If there was, it would already be documented and published.

 

[Pdvdh]

I am not even sure if this is relative to this conversation or not.

Steve

I think it's very relative, from the perspective that you discern the opposite effect. If there was any substantial influence one way or the other, everyone would have the same experience and report the same effect. And if that were the case, we'd all have been informed about the cause and effect many years ago. And every ballistics program would account for the known cause & effect.

In the same manner many account for the affect temperature has on internal ballistics.

 

[RockyMtnMT]

Steve,
The change in pressure over 3,000' El is about 3psi.

The chamber pressure in the cartridge case is around 60,000psi. A 3psi increase or decrease to 60,000psi amounts to a 0.005% change in chamber pressure. Not discernable / measurable.

Not the source of sensed increase or decrease in chamber pressure. Or increased / decreased MV.

I don't think there is a science-based explanation for your observation. If there was, it would already be documented and published.

So based on that our problem was more likely due to a change in temp and a huge change in humidity from NW Montana to central Missouri. I know that it was not low density loading as we always try to get as close as possible to 100% load density as we can. That and having that particular load on the hot rod end when we did it. Good news is it was for family, our customer loads we keep on the mild end.

Steve

 

[mustang58]

I have been waiting for the definitive answer. The only thing I've gotten from this thread so far is a headache.

 

[Edd]

I have been waiting for the definitive answer. The only thing I've gotten from this thread so far is a headache.

The definitive answer is, chamber pressure does not change with altitude.