Range Report Kestrel, pressure, JBM

[Bevan]

Ok so I make my paper drop chart in JBM with a column giving me the additional clicks I need to come down or up on my elevation for every say 20hPa above or below my standard atmosphere data.

I get this data from JBM by changing the "Pressure" field with zero meters altitude and uncorrected pressure

When I am shooting, I change the reference altitude on my kestrel to zero meters above sea level, and then it gives me absolute pressure

Does anyone see any gaping flaws in this, I'm having a hard time thinking tonight.


If I used the corrected pressure from my Kestrel I'd have to do maths to work out the absolute pressure to make the data useful with my chart - ie take off 10hPa per 100meters of altitude above sea level to get my absolute pressure.


So it looks like

RANGE UP 1M/SWIND CLICK PER
METER MIL MIL 20hPa
100 0 0 0
200 0.5 0.1 0
250 0.9 0.1 0
300 1.3 0.1 0
350 1.7 0.1 0
400 2.2 0.2 0
450 2.7 0.2 0
500 3.2 0.2 1
550 3.7 0.2 1
600 4.3 0.3 1
650 4.9 0.3 1
700 5.5 0.3 2
750 6.2 0.4 2
800 6.9 0.4 2
850 7.6 0.4 2
900 8.4 0.4 2
950 9.3 0.5 2
1000 10.1 0.5 2


pressure numbers are made up there. using 140gr amax at 2780fps if you want to run it.

 

[Bevan]

Re: Kestrel, pressure, JBM

ok forum software fucked up the spacing on my chart but you get the idea

 

[Bevan]

Re: Kestrel, pressure, JBM

I could just make like 50 different cards and use density altitude but I don't have a flash kestrel 4500 and I like the idea of a single chart covering all variables.

another column for click per 10*C variation in temperature, a couple of columns on the back for 10,20,30,40* angles, etc

 

[azimutha]

Re: Kestrel, pressure, JBM

If I'm reading this correctly, then switching your Kestrel to corrected pressure means you're going to need to know your altitude +- 50 meters to get good results. My advice is to just use the uncorrected pressures so you can leave altitude always @ zero.

 

[jbmillard]

Re: Kestrel, pressure, JBM

You don't say what calculator you are using to get these numbers, so it's not clear to me whether or not you are correcting for the difference from zero.

Using absolute pressures should work just fine as long as you are consistent.

If you use the trajectory cards they take density altitude so it's a moot point.

Brad

 

[Bevan]

Re: Kestrel, pressure, JBM

Standard trajectory calculator.

Putting together a 'master' card for my rifle to avoid need for PDA in field. Have no way of measuring density altitude.

I use uncorrected or absolute pressure by setting reference altitude of Kestrel to 0

 

[jbmillard]

Re: Kestrel, pressure, JBM

So how are you correcting for zero conditions? Are you calculating the elevation and azimuth at zero conditions and reapplying it?

See here for information on what I'm talking about.

 

[Bevan]

Re: Kestrel, pressure, JBM

Aside from temperature changing MV change between zero conditions/shooting conditions don't change my 100m zero?


MV change from temperature change is corrected for by shooting/chronoing in different weather and logging what the change is, and a column on chart for "click per 10*C"

I use ADI AR2208/9 which are Hodgdon 'extreme' powders and have never displayed a great deal of temperature sensitivity for me, also the temperature range that I shoot in is basically only 0-25 degrees C


Is there something I'm missing here?

 

[jbmillard]

Re: Kestrel, pressure, JBM

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bevan</div><div class="ubbcode-body">Aside from temperature changing MV change between zero conditions/shooting conditions don't change my 100m zero?


MV change from temperature change is corrected for by shooting/chronoing in different weather and logging what the change is, and a column on chart for "click per 10*C"

I use ADI AR2208/9 which are Hodgdon 'extreme' powders and have never displayed a great deal of temperature sensitivity for me, also the temperature range that I shoot in is basically only 0-25 degrees C


Is there something I'm missing here? </div></div>

Zeroing sets the angle between the line of sight and the bore. This doesn't change unless you re-zero. If you shoot in other conditions (velocity, density, temperature, etc) the bullet doesn't cross at the zero point anymore. It may be such a small change that you don't see it, but it's something you should understand anyway. Generally, for short range zeroes and conditions that are close, I doubt you'd see it.

 

[Bevan]

Re: Kestrel, pressure, JBM

Yeah I understand, my last post wasn't very clear

 

[Bevan]

Re: Kestrel, pressure, JBM

with a -100fps muzzle velocity change my 100m zero only moves by .05mil, don't need to correct for that


click per XX*C column takes care of elevation changes further out.. assuming it's a fairly linear change gets you 'close enough' for real world

 

[azimutha]

Re: Kestrel, pressure, JBM

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JBM</div><div class="ubbcode-body">If you shoot in other conditions (velocity, density, temperature, etc) the bullet doesn't cross at the zero point anymore. It may be such a small change that you don't see it, but it's something you should understand anyway. Generally, for short range zeroes and conditions that are close, I doubt you'd see it.</div></div>
From my observations, the change @ 100 yards/meters is generally too small to worry about (and a good reason to use a 100 zero).

So wouldn't it be correct to say that it doesn't matter what conditions are when you zero as long as you zero at a relatively short range for your cartridge? In other words, the bullet is essentially going to travel through the same point @ 100 (maybe minus 1/3 caliber for a .308 Win). You just need to confirm you calculated values with actual range results out at longer distances.

 

[gvanhyning]

Re: Kestrel, pressure, JBM

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bevan</div><div class="ubbcode-body">I could just make like 50 different cards and use density altitude but I don't have a flash kestrel 4500 and I like the idea of a single chart covering all variables.

another column for click per 10*C variation in temperature, a couple of columns on the back for 10,20,30,40* angles, etc </div></div>
I made DA cards and put the data in an Excel file. I have cards for 1K' through 11K' at 2K' increments and it's all on three cards that are laminated in a bag tag from Fedex Office. I have 1k'/3K' on one card, when the data entries are set up to two sheets, you can print one side go to the next sheet, flip the page over and print the other side. I use a standard DA chart that uses altitude and temp to estimate the DA. I have a Kestrel but it doesn't have the DA function.