Barometric pressure vs powder charge

[dat]

Wondering what difference in powder charge shooters have found when traveling to different altitudes, and for this discussion, let's say the temp and humidity stay the same as at home range, but barometric pressure will change in the 1.00 Hg area.

 

[3Sigma]

I wouldn't expect a change in MV for a given charge weight, but the trajectory will be affected due to the differing air density.

 

[Straightshooter1]

Wondering what difference in powder charge shooters have found when traveling to different altitudes, and for this discussion, let's say the temp and humidity stay the same as at home range, but barometric pressure will change in the 1.00 Hg area.

There is a YouTube report suggesting that barometric pressure was effecting his internal ballistics and argued that it was an important factor to allow for (like adding or subtracting kernels of powder to compensate). I didn't feel his method for determining that was really valid; relying on what appears to me to be coincidental relationships. I haven't seen or heard any of any other data relating to this and I remain very skeptical that it has any measurable effect on internal ballistics.

 

[drop_point]

It would seem to me most shooters lack the ability to measure how any variables affect internal ballistics.

 

[dat]

Any short range br guys and or f class guys that have seen group size/shape change with just the change in barometric pressure? Or long range br guys?

 

[gunsandgunsmithing]

Wondering what difference in powder charge shooters have found when traveling to different altitudes, and for this discussion, let's say the temp and humidity stay the same as at home range, but barometric pressure will change in the 1.00 Hg area.

I've not found it to be strictly air pressure. I find temp to be more important but both matter and they follow curves that make the two work together but somewhat independent too. IOW, there are days that you can follow temp alone, for example, but due to air pressure changes, you may or may not stay in tune over the same temp OR pressure change. On top of that, different powders and even loads respond differently to atmospheric changes.

Bottom line, IME there is no absolute way to tune strictly by either pressure nor temp. Figuring out the combined affects and quantifying them... and it might be possible to build a program or even a spread sheet... with enough testing. that with a given powder/cartridge combination, it'll get you close.

 

[AlNyhus]

There was quite a bit of work done by serious 100-300 Benchrest competitors using the DA (density altitude) as a reference for tuning powder charges. DA is basically a composite number that factors in physical altitude above sea level, temperature, humidity, baro, etc. and creates a number that's used as a reference.

With the drag race stuff, we use DA quite a bit along with the amount of water grains in the air for tuning.

 

[Bill Norris]

There was quite a bit of work done by serious 100-300 Benchrest competitors using the DA (density altitude) as a reference for tuning powder charges. DA is basically a composite number that factors in physical altitude above sea level, temperature, humidity, baro, etc. and creates a number that's used as a reference.

With the drag race stuff, we use DA quite a bit along with the amount of water grains in the air for tuning.

Is there a gauge that measures D A ?

 

[gunsandgunsmithing]

Any short range br guys and or f class guys that have seen group size/shape change with just the change in barometric pressure? Or long range br guys?

Yes. Actually, at a match a few years back now, 100-200..a tornadic front came through and the pressure dropped like a rock but temps were steady. The gun went substantially out of tune and I chased it, but finally found it. It was opposite of what I expected, too. Anyway, that was an extreme example that opened my eyes a lot about pressure. Pressure dropping seems to have and offsetting affect to rising temps, which also agrees with what I referred to earlier, that some days a say 20 degree temp change may or may not always take me out of tune, while other times it does.
Hard to replicate the extreme pressure drop as we had at the match I mentioned but I have done it in very similar conditions and it seemed to replicate. I test in more normal conditions more---thankfully. Lol!

 

[AlNyhus]

Is there a gauge that measures D A ?

Bill, I use a Kestrel 5100.

 

[gunsandgunsmithing]

Is there a gauge that measures D A ?

Yes, a Kestrel I think 5100 and maybe another or so but you can usually google up a local weather station and get more precise numbers. I think it matters and is worthwhile but the Kestrel sure is convenient.

 

[gunsandgunsmithing]

Bill, I use a Kestrel 5100.

Beat me to it. That's what mine is as well. That said, I prefer the unadjusted station pressure numbers rather than da.

 

[Straightshooter1]

Is there a gauge that measures D A ?

I record my DA's using a Kestrel Drop-3, which will give me the DA at my location.

 

[Eagle Six]

Wondering what difference in powder charge shooters have found when traveling to different altitudes, and for this discussion, let's say the temp and humidity stay the same as at home range, but barometric pressure will change in the 1.00 Hg area.

Barometric pressure is a rough adjusted pressure better suited for pilots and that is why many of the reporting stations are at/near airports. Density Altitude is used for more precise trajectory calculations because it can be more accurate at your specific shooting location.

 

[Straightshooter1]

Barometric pressure is a rough adjusted pressure better suited for pilots and that is why many of the reporting stations are at/near airports. Density Altitude is used for more precise trajectory calculations because it can be more accurate at your specific shooting location.

Yes. It's good for external ballistics . . . not for internal ballistics since ignition is confined to a sealed enclosed system. . . huh??? ¯\_(ツ)_/¯

PS: The only thing I can think of where DA might somehow effect tune of a load most is any variance for how the air density inside the bore resists the bullet pushing it out of the muzzle, changing the "barrel time"???

 

[LCazador]

I live and shoot in two different climates. I tune differently in both climates and loading ammo in one of these climates and then shooting it in the other is an eye opener!

 

[dat]

Yes. Actually, at a match a few years back now, 100-200..a tornadic front came through and the pressure dropped like a rock but temps were steady. The gun went substantially out of tune and I chased it, but finally found it. It was opposite of what I expected, too. Anyway, that was an extreme example that opened my eyes a lot about pressure. Pressure dropping seems to have and offsetting affect to rising temps, which also agrees with what I referred to earlier, that some days a say 20 degree temp change may or may not always take me out of tune, while other times it does.
Hard to replicate the extreme pressure drop as we had at the match I mentioned but I have done it in very similar conditions and it seemed to replicate. I test in more normal conditions more---thankfully. Lol!

'It was opposite of what I expected' Are you saying it was not 'down and out'?

 

[BoydAllen]

Rhetorical question: Does anyone know of anyone who wins who is able to come to a match preloaded and simply run his tuner based on DA? What about anyone who would claim to hit perfect tune for his first group of a match, based on DA related data?

 

[gunsandgunsmithing]

'It was opposite of what I expected' Are you saying it was not 'down and out'?

Correct, but it's an extreme example and the temp was steady. You don't see that a lot but it's worth remembering.

 

[BP1]

Rhetorical question: Does anyone know of anyone who wins who is able to come to a match preloaded and simply run his tuner based on DA? What about anyone who would claim to hit perfect tune for his first group of a match, based on DA related data?

John Myers