Altitude & Temperature Effects on a Bullet?

[Glock119]

I'll tell ya its a whole lot easier. I use to have to get a ALT reading from my GPS and put that in as a reference for the BAR feature of Kestrel and then had to put both ALT and new adjusted BAR readings along with the others into Exbal. Then I found myself always having to recheck and redo all that info as we changed areas.

Now all I do is set Kestrels BAR reference number to zero and leave it there and leave ALT at zero on Exbal, nothing else to do. Now I just turn on Kestrel and take BAR reading and just put it into palm or laptop so much faster and I think More accurate. /ubbthreads/images/graemlins/laugh.gif

I'd like to thank WARD for his help with this, what a great guy he is. /ubbthreads/images/graemlins/wink.gif

Thanks,
Ben

 

[sierra22]

Shadowman, good to see other people also using pressure.
Your variations for february corresponds very well with what I'm seeing at my part of the globe. (28.43-30.43 "Hg = 962-1030 hPa)

I also use raw/uncorrected pressure or whatever the correct word would be. At present I'm shooting a 6,5mm 120gr SMK as slowly as possibly in order to determine the accuracy of my calculations, the my theory being that a slower bullet will be more adversely affected by this pressure, and thus easier to check.

I also calculate velocity loss due ammo temperature. I freeze a batch of ammo, put it in a cooler bag (to let the ammo slowly warm up) and shoot a few rounds over the chrony at 5Celsius intervals. Not scientifically accurate, but it gives me a fair idea.

I also calculate trajectory based on airtemp, but I'm somewhat uncertain wether this is necessary as I use pressure and ammo temp already

 

[Glock119]

[ QUOTE ]
Shadowman, good to see other people also using pressure.
Your variations for february corresponds very well with what I'm seeing at my part of the globe. (28.43-30.43 "Hg = 962-1030 hPa)

I also calculate velocity loss due ammo temperature. I freeze a batch of ammo, put it in a cooler bag (to let the ammo slowly warm up) and shoot a few rounds over the chrony at 5Celsius intervals. Not scientifically accurate, but it gives me a fair idea.

I also calculate trajectory based on airtemp, but I'm somewhat uncertain wether this is necessary as I use pressure and ammo temp already

[/ QUOTE ]

Thats neat that the Pressure is the same so far away. I would have thought that it might be a little different but its neat to know its not.

I use Temp, Baro, Humitity & (Wind always set at 10 mph to make it easyer to figure 2.5, 5 or 7 mph winds) but I find for most of my shooting that it doese not cause for to much change as localy I am limited to shorter ranges.

But I like to keep track of it for those time when I can go shot at farther distances.

And when ever I get a chance to go some where different I like to take the readings and put them in to see how it effects things compared to home.

By the way what have you found with freezing them and then shooting? any good info produced by doing it or just checking for the heck of it?

Thanks,
Ben

 

[sierra22]

Shadowman, my results from freezing the ammo is far from conclusive yet, as I've only done it 4 times to check powder/primer combination.

With both VitN150 and Norma MRP powder, I get less velocity drop with Fed large rifle primers, than I do with CCI. Accuracy is equal.

With these new "temperature-velocities", I plot them into my ballistic program and see what the it translates to at the various ranges of my dropchart.

For example (I don't have my chart in front of me now - -numbers are out of the blue) if I get 820 meters/second at 15C (standard atmosphere) that translates to 51 clicks (I'm a MIL-man so I don't use MOA) at 600 meters.
Measuring the same ammo at 5C I get 804m/sec which is 55 clicks, or a 4 click deviation. And so on, and so on...

All of this comes together like this:
I'm looking at my ballistic chart which is calculated for standard atmosphere.
I read 1050hPa and 5Celsius of my Silva Alba weatherstation.
Standard atmosphere conditions is 1000hPa and 15C (the ballistic chart).

"Hmm,range is 600 which is 51clicks. Deviation from SAC is 50hPa and 10Celsius. My chart says at this range says 0,2 clicks per 10 hPa and 0,4 clicks per 5Celsius and +4 for ammo temperature. That is +1 click for hPa and +0,8click for temperature. 51clicks + 1 + 0,8 +4 = 56 clicks corrected.

Not 100% accurate, but a lot more than not doing it, or so it seems.

 

[Glock119]

I have never much been a fan for the mil thang, as I could never figure it out. I guess there right when they say you can't teach and old dog new tricks. /ubbthreads/images/graemlins/shocked.gif

Also if I can ask what ballistic program are you using?

Thanks,
Ben

 

[sierra22]

I use a free program called PCB 1.8. It's available on the net and will give both metric or imperial (correct name??) units. It works reasonably well at the shorter ranges (600minus) as long as I juggle Bc to something that corresponds ok to what the bullet really flies.

It might be helpful to know that a loss off 1hPa or 0.02953"Hg corresponds to about 27m or 30m (cannot remember which right know) increase in altitude under standard atmosphere conditions.. (app 88 or 98.5ft)

MOA and MIL... well, it's an aquired taste which way you'll swing I guess.
As long as you have a metric scope (1cm@100m) everything is dividable by 10. ¨
What will 56clicks translate to in holdover? 5,6 MIL.
Good, simple math for a tired head...