Case over pressure and altitude.

[SpeedGoatShooter37]

Hello all. Before I get started, yes, I've searched the ends did the internet looking for a comprehensive answer to this question.

I was bit by the reloading bug several years ago when I was stationed in Rapid City. Elevation there is about 3300 feet. About a year ago I was re-assigned to NW Louisiana, where the elevation is about 300 feet.

Not once did I ever experience case over pressure while in Rapid City. No case flow, no sticky bolt, no cratered or flat primers, and no popped primers. As soon as I moved down here I've had it in three guns. The only, but undeniable, sign is a perfect circle from case flow into the plunger. My Savage 30-06 (the worst offender) and my 22-250 and 7mm-08 in 700s all seem to b sensitive to it now. New brass to boot.

The only variable that has changed is altitude and to a degree humidity. I'm shooting my same supply of powder that I had up there in very similar temperatures. Can someone please teach me the science as to how a slight change in the mercury can cause this, or give me a second opinion to the cause?

 

[CatShooter]

You have given almost no information - load, powder, bullets, rifles, temperatures in Rapid City and NW Louisiana, were the loads max in Rapid City?

Common, please give us something to work with...

 

[Trnelson]

That is a viable explaination. At lower elevation the atmosphere is more dense in terms of molecules per unit volume. Greater medium density, in this case the atmosphere, will yield more pressure (generally).
ICAO standard atmospheric pressure at sea level is 29.92"HG (at standard temperature). At 3,000 MSL standard atmospheric pressure is 26.8" HG (at standard temperature. In aviation we use a rule of thumb of 1" less mercury for every 1,000' up, you can see that holds pretty close, provided the temperature is the same.
Here is a table that lists standard pressure up to 10,000' MSL. It is an excerpt from here Table 1-6.U.S. Standard Atmosphere Heights and Temperatures . More indepth explaination exists, but for your purposes this should provide you a pretty good idea as to what you experienced.
Sea Level 0' 29.9" 100.0%

1000'305 28.9"733 14.2 97.7%
1500' 457 28.3"720 13.9 96.0%
2000'610 27.8"707 13.7 95.4%
2500' 762 27.3"694 13.4 92.5%
3000 914 26.8 681 13.2 90.8%
3500 1067 26.3 669 12.9 89.1%
4000 1219 25.8 656 12.7 87.4%
5000 1524 24.9 632 12.2 84.3%
6000 1829 24.0 609 11.8 81.2%
7000 2134 23.1 586 11.3 78.2%
8000 2438 22.2 564 10.9 75.3%
9000 2743 21.4 543 10.5 72.4%
10000 3048 20.6 523 69.7%

 

[CatShooter]

That is a viable explaination. At lower elevation the atmosphere is more dense in terms of molecules per unit volume. Greater medium density, in this case the atmosphere, will yield more pressure (generally).
ICAO standard atmospheric pressure at sea level is 29.92"HG (at standard temperature). At 3,000 MSL standard atmospheric pressure is 26.8" HG (at standard temperature. In aviation we use a rule of thumb of 1" less mercury for every 1,000' up, you can see that holds pretty close, provided the temperature is the same.
Here is a table that lists standard pressure up to 10,000' MSL. It is an excerpt from here Table 1-6.U.S. Standard Atmosphere Heights and Temperatures . More indepth explaination exists, but for your purposes this should provide you a pretty good idea as to what you experienced.
Sea Level 0' 29.9" 100.0%

1000'305 28.9"733 14.2 97.7%
1500' 457 28.3"720 13.9 96.0%
2000'610 27.8"707 13.7 95.4%
2500' 762 27.3"694 13.4 92.5%
3000 914 26.8 681 13.2 90.8%
3500 1067 26.3 669 12.9 89.1%
4000 1219 25.8 656 12.7 87.4%
5000 1524 24.9 632 12.2 84.3%
6000 1829 24.0 609 11.8 81.2%
7000 2134 23.1 586 11.3 78.2%
8000 2438 22.2 564 10.9 75.3%
9000 2743 21.4 543 10.5 72.4%
10000 3048 20.6 523 69.7%

That does not explain or account for his high pressure signs.

 

[Trnelson]

That does not explain or account for his high pressure signs.

I would beg to differ sir. Assuming that the OP is in fact shooting the same stock of components in the same rifles, as he states he is. He further states, at a very similar air temperature.
Possibly, he's not giving us accurate information in an effort to start a keyboard ******* match. I have no way to confirm or deny that and have to limit my answer to assuming he is providing accurate information. Maybe he is, maybe he isn't.
That being stated, I can report that I just Saturday witnessed my friend go through the same issue. He lives at about 5,000' and shot here with us at 1,300'. His load using H4350 shoots with no indication of adverse pressure at his place. I have personally witnessed him shoot the rifle and load at his place and have shot it myself. He had a slightly sticky bolt lift on Saturday while here. He fired a total of three rounds before we thought it better to just put his rig away and enjoy the other toys on hand. He drove back home checked over the rifle and broke down a couple pieces of his ammo. He confirmed they were correct, re-seated the bullets and had no issue firing the ammunition at his place.

 

[CatShooter]

I would beg to differ sir. Assuming that the OP is in fact shooting the same stock of components in the same rifles, as he states he is. He further states, at a very similar air temperature.
Possibly, he's not giving us accurate information in an effort to start a keyboard ******* match. I have no way to confirm or deny that and have to limit my answer to assuming he is providing accurate information. Maybe he is, maybe he isn't.
That being stated, I can report that I just Saturday witnessed my friend go through the same issue. He lives at about 5,000' and shot here with us at 1,300'. His load using H4350 shoots with no indication of adverse pressure at his place. I have personally witnessed him shoot the rifle and load at his place and have shot it myself. He had a slightly sticky bolt lift on Saturday while here. He fired a total of three rounds before we thought it better to just put his rig away and enjoy the other toys on hand. He drove back home checked over the rifle and broke down a couple pieces of his ammo. He confirmed they were correct, re-seated the bullets and had no issue firing the ammunition at his place.

What you are saying is anecdotal... it does not explain his pressure signs or problems.

 

[Bravo 4]

I have never had this problem and I've shot in about as many different environments as the planet can throw at ya. I currently live and develop my loads at around 300 feet asl, I elk hunt at 8,000+.
I'm not particularly a fan of anyone but this fella is more of a subject matter expert than I ever will be, so I tend to listen:

 

[Bravo 4]

I think you have other issues. Like maybe you cleaned your rifle and left some cleaner/oil in the chamber, or your powder scale got bumped on the interstate move and needs checked.

 

[memtb]

I'll go with greater air density as well as, in all likelihood, much higher humidity. "IF", you were right on the edge of max. pressure, this may be enough to push you over! memtb

 

[Mikecr]

I don't know the cause here, but I'll throw a notion into it that I haven't even proven to myself..
Maybe the powder settled in cases denser toward primer. This, dependent on your load density to begin, and the Rh/dewpoint present during loading.

With a load density below ~90%, you can change MV with a gun pointed straight up -vs- straight down. Try it sometime. At the range, with witnessed consistent MV across your chronograph, chamber a round, lift the gun off bags & point it straight up. Now carefully lower the gun to level on your bags and fire. MV can be higher(+20fps). Do the same again, but after pointing the chambered round straight down. MV can be lower here(-20fps).
I come to this after misses on groundhogs in PA mountains. A lot of up & downhill shooting there, and I account for slope shooting ballistically, but there was something else contributing. After seeing it with mentioned test, I have not accepted load combinations causing <95% load density per QuickLoad.

Traveling with ammo pointed up leads to settling I'm sure, but it should not affect anything with loose powder kernels. I wonder if powder could hold enough moisture that with traveling it might sort of cake into a slug near webs. Try holding a suspect by the case head and rap it against your knee a few times before shooting. Does it make a difference?

 

[mechengr]

Hello all. Before I get started, yes, I've searched the ends did the internet looking for a comprehensive answer to this question.

I was bit by the reloading bug several years ago when I was stationed in Rapid City. Elevation there is about 3300 feet. About a year ago I was re-assigned to NW Louisiana, where the elevation is about 300 feet.

Not once did I ever experience case over pressure while in Rapid City. No case flow, no sticky bolt, no cratered or flat primers, and no popped primers. As soon as I moved down here I've had it in three guns. The only, but undeniable, sign is a perfect circle from case flow into the plunger. My Savage 30-06 (the worst offender) and my 22-250 and 7mm-08 in 700s all seem to b sensitive to it now. New brass to boot.

The only variable that has changed is altitude and to a degree humidity. I'm shooting my same supply of powder that I had up there in very similar temperatures. Can someone please teach me the science as to how a slight change in the mercury can cause this, or give me a second opinion to the cause?

You say you used new brass? Are you saying the new brass was shot in Louisiana? If you worked up brass to near max load with fireformed brass and now using new, or if you happen to be using brass with a different head stamp than you worked up with in Rapid City, could easily explain it. I have a hard time believing a few inches of mercury pressure change in atmospherics has enough effect on internal ballistics to push a load over the edge.

 

[SpeedGoatShooter37]

You have given almost no information - load, powder, bullets, rifles, temperatures in Rapid City and NW Louisiana, were the loads max in Rapid City?

Common, please give us something to work with...

I respectfully disagree. The only variable that has changed is elevation. However since you asked:

30-06: Nosler brass, 190 grain Accubond Long range, over 60 grains of RL-22 ( same can I had in Rapid), with a CCI primer, set to 2.800" Difference in outside air temperature was 5 F.

22-250: Winchester brass, 50 grain VMax, 34.5 of Varget, set to 2.350" Again difference in air temp was 5 F. 82 in NW Louisiana vs 77 in Rapid.

In neither instance nor location were the loads in the sun or left in a hot vehicle.

I truly hopes this does give you enough infor to diagnose the cause.

 

[SpeedGoatShooter37]

Possibly, he's not giving us accurate information in an effort to start a keyboard ******* match.

I am new to the forum and given the dumpster fire that has become the internet, I understand your suspicions. Rest assured I have no intention of starting a ******** match. We all have better things to do with our time.

I'm simply trying to diagnose the issue. Work doesn't allow me as much time as I'd like for load work up. I think we can all agree that any knowledge that helps one become more efficient and better comprehend the variables at play is a good thing.

 

[gohring3006]

Puzzling.....

But the obvious route here is a new load work up.

I always tell myself,
"When in doubt, work back up".