Tuning for different altitude.

R

rooshooter

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Tuning for different altitude.(Density Altitude)

If a rifle was in tune at sea level, then you went up in altitude, say to 6000 feet, generally speaking what way would you need to adjust the powder charge if the temperature stays the same. Not interested in elevation settings, just keeping the same tune/velocity
 
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Muzzle velocity is not going to change with altitude. It's the downrange ballistics that will. You can run some tables with a program like JBM to see the trajectory differences. I'm not sure how you could maintain a "tune" , except the way you do it for any other condition change. The old fashioned way, that is, tryin' it out.:D

Ray
 
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So, when the benchresters are talking about density altitude and adjusting powder and tuner settings, what are they adjusting for?
 
rooshooter

I'm not sure I understand your question. If you are wanting some sort of hard numbers, such as, if you increase the altitude by 500', you go up two clicks on the powder, then I'm not sure there is such a thing. All of the shooters I know tune more by instinct and experience, and by firing groups to verify the adjustments.

Without access to a range, I don't know of any way to tune LR loads for altitude/density/etc, or for accuarcy. For LR Benchrest, I load at home, usually 200 +/- rounds at a time and I only adjust for altitude by adjusting velocity based on which range I'm going to shoot at. For example, a 6000' elevation difference between, say, Sacramento and Raton.

But, most of my shooting buddies are lousy shots like me so what do we know? Hopefully, somebody here will chime in and bail me out.

Ray
 
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Cheech, not hard numbers, all calibers would be different. Just, for higher altitude, go up a tad in the powder load or down or what ever.
 
If it means the difference between shooting .100" groups at 100 and shooting .095" groups, it doesnt sound like it will do me much good. I do change my loads slightly between summer and winter loads if I know it will be a big temperature difference, but have not shot at altitude for a while now.
 
rooshooter -

Maintain one velocity level ................. Meaning no matter what the air density, altitude, temperature, etc. you adjust the load for the present ambient.
I call it "OBV" (Optimal Barrel Velocity) and is how I load my match ammo. How I derive at what charges to use takes a lot of testing over a wide range of ambient and powder sensitivity testing to your specific cartridge/powder/primer/bullet combination.

Here is a example image of a Temperature Sensitivity chart that I made and use for one of my rifles:

DasherChart.jpg

Typically I load at home to a forecast of an event, or I load in the Motel the night before, again to the forecast. I don't care if I "hit the nail directly on the head" but am trying to keep my velocity with in a velocity window of the accuracy node.
I have loaded heavier charged rounds for cooler morning forecasts and then lighter rounds for the warmer afternoons, etc., etc. .........

In closing: as I wrote above, it takes testing and is VERY cartridge and powder specific, besides the other aspects.

Happy Shooting
Donovan Moran
 
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I think your biggest problem at 1,000 yards is actually being able to tell the difference between minute changes in loads on the target? There are so many variables that you never know if it was the load or a condition you could not see? For me, I would stick with a load that you can tune at say............200 to 300 yards. When you get to your new altitude, is just a matter of "zeroing-in?".
Rich De
 
I believe that load adjustments for air density are valid -when you need to hold the precision tune the point blank BR folks are competitive with.
The piston/bullet is pushing out a slug of air which can vary in weight, just as a bullet might vary in weight. This could affect burn rate, and barrel timing just a hair. Enough to mis-shape otherwise 0.1moa grouping.

But distant performance is more a managing of external ballistics, than internal.
You don't get slumpy over 0.05moa worse grouping at 1kyd, from an air density tune change, cuz you don't even know it.
 
mikecr said:
The piston/bullet is pushing out a slug of air which can vary in weight, just as a bullet might vary in weight. This could affect burn rate, and barrel timing just a hair. Enough to mis-shape otherwise 0.1moa grouping.
Click to expand...

Do you have any data to support this? The mass of air in a typical barrel weighs less than 0.5 grains, and even going from sea level to 6000 feet or a change in temperature by 100 F, it varies by only about 20% or 0.1 grains. Do you really think that 0.1 grain is going to make a 0.1 moa difference in the internal ballistics? I have found that a variation in bullet weight of even 1 grain doesn't make much difference.

Cheers,
Keith
 
your willing to shoot plus or minus 0.5 gr bullets in competition ?.......
even sierra does not build that loose in thier production bullets(plus or minus 0.3)
i wont shoot plus or minus .2 in competition.
seems like a strange challenge to me.
mike in co
mks said:
Do you have any data to support this? The mass of air in a typical barrel weighs less than 0.5 grains, and even going from sea level to 6000 feet or a change in temperature by 100 F, it varies by only about 20% or 0.1 grains. Do you really think that 0.1 grain is going to make a 0.1 moa difference in the internal ballistics? I have found that a variation in bullet weight of even 1 grain doesn't make much difference.

Cheers,
Keith
Click to expand...
 
I have absolutely no 'data' to prove it.
My notion came about from years of reading in the poink blank BR sections, where there have been shooters that adjust their loads or tuners for changes in DA.

I don't, and won't ever shoot so well as to prove it.
 
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