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# Pressure/temp vs trajctory

#1
07-23-2004, 05:36 AM
 Silver Member Join Date: Dec 2002 Location: Norway Posts: 126
Pressure/temp vs trajctory

Could use a bit of help with my problem, as I'm somewhat a math-lectic.

PROBLEM: Correcting trajectory for measured barometric pressure and temperature to score first round hits without sighters.
Trajectories are to my understanding calculated according to a standard atmosphere. Change this and the bullet will behave differently and the calculated BC will change.

SO FAR: If barometric correction X temperature correction X original BC = new BC
and
Barometric corr = Standard pressure : measured pressure,
Temp corr = measured Rankin temp : standard Rankin temp

...then I can calculate the deviations (not correctly, but better than not doing it) by working out a new dropchart for the new BC.

I WANT: To find correction factors where I can correct directly from a trajectory chart without going the long way around and making a new one.

Any takers ?
[img]images/icons/confused.gif[/img]

[ 07-23-2004: Message edited by: sierra22 ]
#2
07-23-2004, 07:38 AM
 Silver Member Join Date: Aug 2003 Posts: 126
Re: Pressure/temp vs trajctory

Most programs allow you to input environmental variables and generate a new trajectory without changing the BC. The program just changes the air density and speed of sound accordingly.
Try JBM's ballistic site.
#3
07-23-2004, 10:46 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 4,227
Re: Pressure/temp vs trajctory

Your looking for a way to apply a percentage to your bullets path for a difference in air density?
Like if today's temp is the same, but a front is coming, and today's pressure is 1" of mercury lower. How will that effect my drop today?
Dude, that would be a seriously ugly formula to carry in the field.
However, various common scenarios could be run on a program, and then a list of rough corrections could be made & carried along.

[ 07-23-2004: Message edited by: Mikecr ]
#4
07-23-2004, 03:54 PM
 Silver Member Join Date: Dec 2002 Location: Norway Posts: 126
Re: Pressure/temp vs trajctory

Yes, you all understand the intention with my question correctly. Complicated yes, but satisfactory fun if I can get it working.
True, sighters or several charts will take care of (parts of) the problem.

However, I know hard target interdiction operators correct their MOA on a "fill-in-the-blanks" formula using constants calculated for the various ranges, AND hitting their targets.

I have the data for the Raufoss .50 MP ammunition and I'm familiar with using the form and the calculator, but I don't understand the process of finding the variables that would be valid for other calibers than the .50.

To illustrate:
In the past I calculated a chart for the 600m range I sometimes use, but it was never "quite right" even though very, close. In winter it wasn't all that close.
On a mild winter day I'd get 52 instead of 48 clicks (1/3 MOA scope) for elevation. This I accredited to reduced powder burn rate.
For windage I could get about 41 instead of 37 clicks for windage (almost constant wind from the sea), which I accredited to poor wind calling.
Of course these errors would get larger with range, but 600m is all I have collected data for.
However, when I was taught some neat tricks and borrowed a wind/barometer/termometer, calculated the new BC it all got quite a bit closer.

A calculation could look like:
(stand. pressure: measured press.)x(m. temperature:stand. temp.)x manufacturers calc. BC
(1000:980)x(480,9:517,7)x0,458= 0,433 (BC corrected for pressure (in hPa, works same way for inches Hg if I understand the theory correct) and temperature (simple temperature measurement converted to Rankine).

[ 07-23-2004: Message edited by: sierra22 ]

[ 07-23-2004: Message edited by: sierra22 ]

[ 07-23-2004: Message edited by: sierra22 ]
#5
07-23-2004, 11:13 PM
 Writers Guild Join Date: Jul 2001 Posts: 1,459
Re: Pressure/temp vs trajctory

This really isn't a cop out but the best way to account for all variables on the day you point your muzzle at your target game.

At long range, there are so many variables that simply can't be calculated out if you want to be certain that you can hit your target.

The safest is take a spotter shot or two a few hundred yds from your game. Adjust as necessary. Get on the target.

It only takes a few more seconds, confirms everything, and doesn't disturb the game.

At ranges 700 to 1700yds, this helps me to be sure.

Jerry
#6
07-24-2004, 06:57 PM
 Silver Member Join Date: Dec 2001 Location: bluefield,va,us Posts: 339
Re: Pressure/temp vs trajctory

how about perry systems on a palm.it works.
__________________
keith
#7
07-26-2004, 10:36 AM
 Silver Member Join Date: Jun 2002 Location: Ireland Posts: 138
Re: Pressure/temp vs trajctory

Sierra22

Send me a message email by clicking on the first envelope symbol above.

I will prob. be able to help you out with charts that you can print off and bring to the range with you.

I will need to get basic ballistic data from you.

Kindest regards

700

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