Chamber pressure vs. altitude

[Tom Smith]

Does altitude affect chamber pressure? If a near max load is fine at sea level, will it be over pressure at a higher altitude? Or the other way around, if you develop a load at say 8,000 ft., will you see higher pressures if you shoot the same load at sea level. Or is chamber pressure the same regardless of altitude?

 

[#40Fan]

Higher the altitude, the lower the pressure.

 

[chkunz]

If you load to 50,000 psi at sea level where the ambient pressure is 14.7 and then shoot on the moon where the ambient pressure is ~0 then you will shoot with an ambient pressure that is ~14.7 psi lower than your sea level load. If your sea level load produced 50,000 psi then substract 14.7 to your estimated sea level pressure of 50,000 psi for your moon shot. Not significant.

 

[Webster]

If you load to 50,000 psi at sea level where the ambient pressure is 14.7 and then shoot on the moon where the ambient pressure is ~0 then you will shoot with an ambient pressure that is ~14.7 psi lower than your sea level load. If your sea level load produced 50,000 psi then substract 14.7 to your estimated sea level pressure of 50,000 psi for your moon shot. Not significant.

Don't want to even understand sea level stuff. The pressure is generated in the space behind the bullet in the bore, a sealed volume. -14.7 sounds reasonable.

 

[SmokinJoe]

I think there is more to this than you may be thinking, but I am no expert. Seems like the density of the air in the bore probably comes into play also.

 

[chkunz]

I think there is more to this than you may be thinking, but I am no expert. Seems like the density of the air in the bore probably comes into play also.

If you have a cartridge that will produce 50,000 psi in a vacuum you can expect a few psi higher at one atmosphere but the difference is small and can be neglected for our purposes.

 

[klmshooterk]

I went through an extensive exercise last year to make a drop chart for 8000ft for my rifle sighted in at 300ft and 80 degress. Once I got done accounting for altitude/air pressure, temp effects (powder), I found that the effects counter-acted each other and I didn't even bother using them to adjust my drop chart until beyond 700 yards.

I test-fired when I got to altitude, in cooler fall conditions, and was right on. Of course if you have a powder of greater temp sensitivity YMMV.

 

[Liven The Dream]

For what it's worth ??? I shoot Palma and 1000 Prone
300, 500,600,800,900 and 1000 yards.
I live at just over 4000 F EL . I load and test at this elevation …
I have shot at Raton 7600 F EL and Sac. Ca. at around 2-3 hundred F EL and many other Ranges .

My Ammo works all over …
I will tell you at 7600 with thin Air at 1000 yards you use less elevation on your sights...

 

[Webster]

I think there is more to this than you may be thinking, but I am no expert. Seems like the density of the air in the bore probably comes into play also.

60,000 PSI minus 14 PSI?

 

[KenOath]

More air density may make a small difference to pressures in chamber but I believe it has a bigger effect on Bullets in flight to target, you will need to crank up elevation on scope more the closer you get to sea level..

 

[dcali]

No. There is no signficant difference (not even close). Powder does not require or use the atmosphere in combustion, and atmospheric pressure even at sea level is a tiny percentage of chamber pressure. And the air in front of the bullet does not generate significant resistance.

External ballistics is another matter, but chamber pressure will be practically identical and not change with altitude.

 

[dellet]

It can depend on other conditions that often go along with elevation changes.

You are likely to have a temperature change with an 8000 foot change in elevation between load development and actual shooting, and you would be more likely to see a difference going from high elevation to low which generally has an increase in temps, but there are some powders that also get erratic when cold.

If you have had an issue, it would be worth looking at the whole picture.

 

[Flouncer]

This would very easy to test and verify. Just need a tank of gas and some ambition.

 

[GSPV]

For practical purposes, internal ballistics are not affected by external pressure or humidity (more on that in a minute). Internal ballistics *are* directly affected by temperature. There is a direct, *and **locally** linear*, relationship.

*External* ballistics, OTOH, are definitely affected by atmospheric density. Altitude is one component of atmospheric density.

WRT internal ballistics, humidity and temperature:

1. Weight of powder is the best approximation we have for how much energy is in a load of powder.

2. Density of powder (weight per unit of volume) is *greatly* affected by humidity. Soooo, if you drop powder while at a match, you need to be aware that the weight of powder at a given setting will change throughout the day. 52 clicks might be 30.3 grains in the morning and 29.7 in the afternoon.

You don’t have to believe me about this. I hope that you won’t and will go do the research yourself.