Range Report A comparison of ballistic programs vs actual Dope

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Use the altimeter setting from the airport, but put in the altitude of your actual shooting location, and tell it to correct that for standard atmosphere.

It's not as precise as setting in the station pressure, temperature, and relative humidity, but it will be close.

Note the comments about altimeter settings in this link:

http://wahiduddin.net/calc/pressure.htm

You don't necessarily need to carry a Kestrel - the pressure sensor is my Casio Pathfinder watch is quite accurate. However a Kestrel 4000 or higher is nice, because it will calculate accurate density altitudes for you, in case you want to use a density altitude table in preference to carrying a ballistic computer in the field. See:

Making Ballistic Cards Using Density Altitude

But note that JBM will now calculate DA tables for you:

http://www.jbmballistics.com/cgi-bin/jbmcard-5.0.cgi

 

[LoneWolfUSMC]

Re: A comparison of ballistic programs vs actual Dope

While we are on the topic, here is something that has confused me about the Kestrel.

I use a 2500NV. I know that I can get station pressure by setting the reference altitude to "0".

I know that to get the correct altitude you have to set the barometric pressure reference.

If you just materialized out of nowhere onto a mountain and had no idea what your barometric pressure was....how the hell do you figure your altitude with the Kestrel? It seems that you have to know one or the other.

So far I have just been letting Ballistic query the WX station and auto set the environmentals. My area is pretty flat and elevation doesn't change more than 50' for quite a ways. When i do get up into the hills I will probably just set the station pressure and drive on.

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If you just materialized out of nowhere onto a mountain and had no idea what your barometric pressure was....how the hell do you figure your altitude with the Kestrel?</div></div>

You can't.

The Kestrel has only a pressure sensor.

If you tell it the local barometric pressure referenced to sea level, which you set in the altitude screen, it can then calculate the altitude.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">It seems that you have to know one or the other.</div></div>

Exactly.

Assuming that you are running ballistic software in the field, set the reference altitude in the Kestrel pressure screen to 0. That then displays the absolute station pressure. Enter that, the temperature, and the relative humidity into your software, for the most accurate calculation of air density.

If you are in the mountains, but are somewhere you know the altitude from a map or other source, you can find the current sea-level-referenced barometric pressure by going to the Kestrel altitude window. Change the BP reference in that window until the display altitude matches where you are.

Now you can read the displayed altitude as you move around - until the BP changes.

I do not recommend using a Kestrel for altitude, because without an external reference, you have no way of knowing the SL BP. If you want to know the altitude, use a GPS.

That's why pilots getting ready to take off set their altimeter to the local field altitude as a reference. When they are landing, the tower tells them the SL BP, so they can correctly read their altitude.

 

[Rich Emmons]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">
But note that JBM will now calculate DA tables for you:

http://www.jbmballistics.com/cgi-bin/jbmcard-5.0.cgi


</div></div>

Lindy, how come JBM's elevation values go just slighty up with temperature and every other table I've seen are correctly the opposite?

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Rich: I don't know what you're seeing. If you think there is an error with JBM, the output screen has a link for reporting that.

 

[Rich Emmons]

Re: A comparison of ballistic programs vs actual Dope

1000 ft ------------------ 6500ft
Range (yd)40°F60°F80°F ----40 °F 60 °F 80 °F Range (yd)


1000 -38.8-39.0-39.2 ----33.1 -33.3-33.5 1000

That's what I'm seeing.

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Rich:

Without being able to see the complete input screen on whichever of JBM's several calculation programs you're using, I can't make sense of that output.

It's probably an input error of some kind, as I've found JBM to be pretty reliable.

Make sure that you don't have the "Standard Atmosphere at Altitude" box checked.

If you think there is an error, report it.

 

[azimutha]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">
Make sure that you don't have the "Standard Atmosphere at Altitude" box checked.
</div></div>

Should that always be the case? I've never checked it.

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

JBM Atmospheric Parameters

This applies to the standard trajectory screen, http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi

If you check "Standard Atmosphere at Altitude", you get the ICAO temperature and pressure at whatever altitude you specify, with the humidity set to zero. That means that whatever temperature, pressure, and relative humidity you might have entered in the input screen are completely ignored. See the chart of the ICAO Standard Atmosphere below. The temperature and pressure listed for the appropriate altitude in that chart are what the program will use.

If "Standard Atmosphere at Altitude" is not checked, temperature and RH entered in the appropriate boxes always affect the air density calculation.

If "Pressure is corrected" is checked, the density is calculated using the pressure corrected for whatever altitude is entered in the altitude box. It means that the pressure you entered is from a source which corrected the station pressure to the sea-level-referenced barometric pressure, and the program corrects <span style="font-style: italic">that</span> pressure for altitude.

That's confusing to me, but I didn't write the program. It still uses the temperature and relative humidity entered in the appropriate places.

If "Pressure is corrected" is not checked, the density is calculated ignoring the contents of the altitude box. In other words, the programs assumes that you have entered the station pressure. It still uses the entered temperature and relative humidity in the air density calculation.

The most accurate way to get any ballistic program to work is to have an instrument like a Kestrel or many watches which measures local atmospheric pressure, which we call the station pressure. Enter the station pressure, the temperature, and the relative humidity, and make sure that you're not using a program option to calculate the "standard atmosphere".

 

[DaveV]

Re: A comparison of ballistic programs vs actual Dope

These are the only atmospheric inputs on this page:

JBM - Calculations - Trajectory Card

Low Temperature (-40.0 to 140.0 °F) [32.0] [?]
High Temperature (-40.0 to 140.0 °F) [70.0] [?]

Zero Altitude (-1000.0 to 15000.0 ft) [0.0] [?]
Zero Temperature (-40.0 to 140.0 °F) [59.0] [?]

I assume that the program is calculating Density Altitude using Zero Altitude and Zero Temperature. It must, therefore, assume standard station pressure and humitity as constants.

Am I getting this correct, Lindy?

 

[JT505]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">JBM Atmospheric Parameters

If "Pressure is corrected" is checked, the density is calculated using the pressure corrected for whatever altitude is entered in the altitude box. It means that the pressure you entered is from a source which corrected the station pressure to the sea-level-referenced barometric pressure, and the program corrects <span style="font-style: italic">that</span> pressure for altitude.

That's confusing to me, but I didn't write the program. It still uses the temperature and relative humidity entered in the appropriate places.</div></div>
Lindy,
I find it (JBM) confusing also.

The software should be accounting for Temperature; user entered or calculated (ICAO) to make calculations. Is the "Pressure is corrected" check box correcting user entered pressure altitude adjusted for non-standard temperature, producing density altitude?

Thanks,
Jim

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

Lindy, your post above finally made the lightbulb click on for me about using JBM and which boxes to check for what data you have entered. Thanks for that.

A related question to your previous reply to me:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Use the altimeter setting from the airport, but put in the altitude of your actual shooting location, and tell it to correct that for standard atmosphere.

It's not as precise as setting in the station pressure, temperature, and relative humidity, but it will be close.

Note the comments about altimeter settings in this link:

http://wahiduddin.net/calc/pressure.htm</div></div>

If I'm shooting in the mountains and I'm taking a 1K+shot at a target above or below me by a significant change in altitude, say > 1000 feet - what altitude has a greater effect on the ballistic calcs? The target elev or the shooter's elev? I know the angle calcs change the drop slightly based on a pure trajectory issue, but the DA will be different at each location and should have an effect as well. Obviously I probably won't know my target's elev unless I do the Trig based on the range and angle. But for sake of discussion, is there enough of a effect on DA differences to take into account?

I know that's getting into the weeds on ballistic theory, but I'm just trying to fill my knowledge bag. Thanks!

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Jim: long time no see! Cheers!

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Is the "Pressure is corrected" check box correcting user entered pressure altitude adjusted for non-standard temperature, producing density altitude?</div></div>

Ahh, you pilots!

Not exactly. Pressure altitude is what the altitude would read if the sea-level-barometric pressure were 29.92 inches. The program doesn't know what pressure altitude is. It just uses whatever altitude you enter. It might be from a map, a GPS, or a wild-ass guess.

But the program does attempt to correct the entered pressure for the user-entered altitude if that box is checked, and it does use the user-entered temperature and relative humidity in the density calculation.

Natually, that pressure calculation is an approximation, which is why it's better to enter the actual station pressure, if you know it, and leave that box unchecked.

 

[DaveV]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DaveV</div><div class="ubbcode-body">These are the only atmospheric inputs on this page:

JBM - Calculations - Trajectory Card

Low Temperature (-40.0 to 140.0 °F) [32.0] [?]
High Temperature (-40.0 to 140.0 °F) [70.0] [?]

Zero Altitude (-1000.0 to 15000.0 ft) [0.0] [?]
Zero Temperature (-40.0 to 140.0 °F) [59.0] [?]

I assume that the program is calculating Density Altitude using Zero Altitude and Zero Temperature. It must, therefore, assume standard station pressure and humitity as constants.

Am I getting this correct, Lindy?

</div></div>

In playing with the program some more, I found that the only parameter that would effect the output is BC. The parameters stated above have no effect on output. Therefore, the parameter

"Altitudes (Comma separated list of up to 10 altitudes) (-1000 to 15000 ft) [?]"

gives Density Altitude based on internal constants.

Yes?

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Dave:

JBM has several calculation pages. If you'll post the actual web link to that page, I'll try to answer your question.

 

[DaveV]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">Dave:

JBM has several calculation pages. If you'll post the actual web link to that page, I'll try to answer your question.
</div></div>

http://www.jbmballistics.com/cgi-bin/jbmcard-5.0.cgi

Thanks!!

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If I'm shooting in the mountains and I'm taking a 1K+shot at a target above or below me by a significant change in altitude, say > 1000 feet - what altitude has a greater effect on the ballistic calcs? The target elev or the shooter's elev? I know the angle calcs change the drop slightly based on a pure trajectory issue, but the DA will be different at each location and should have an effect as well. Obviously I probably won't know my target's elev unless I do the Trig based on the range and angle. But for sake of discussion, is there enough of a effect on DA differences to take into account?</div></div>

Actually, that's a really good question.

Several ballistics programs will calculate angled shots.

Not a single one that I know of will take into consideration the difference in air density between the shooter's position and the target.

What I would do if the difference is significant - 1000 feet is probably not enough - is to average the firing position altitude and the target altitude, and use that to compute the shot.

And if you're this far into the theory, I think you really need to get a copy of Bryan Litz's book.

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Dave:

That page is pretty complicated.

That section is used to try to calculate the change in your muzzle velocity with temperature. If you don't have data for that, put in the muzzle velocity you know in both boxes, and put the same temperature in both boxes.

The calculation of the air density at each density altitude step you specify uses the air temperature and pressure of the ICAO Standard Altitude I referenced above.

 

[DaveV]

Re: A comparison of ballistic programs vs actual Dope

That's what I concluded. Thanks! By the way, how did you copy that window and get it to display here so well?

 

[blackwidow2912]

Re: A comparison of ballistic programs vs actual Dope

i am trying to understand this. i use bulletflight on my ipod touch. if i use the baro pressure from my bruton wind meter should i also enter the altitude or should i leave it at zero. i am getting different dopes for each. sometimes i have a thick skull i just need to pound this in so i understand

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: blackwidow</div><div class="ubbcode-body">i am trying to understand this. i use bulletflight on my ipod touch. if i use the baro pressure from my bruton wind meter should i also enter the altitude or should i leave it at zero. i am getting different dopes for each. sometimes i have a thick skull i just need to pound this in so i understand
</div></div>

I'm sure Lindy or others will bitch slap me if I'm wrong - but if I paid attention correctly to what I've read here - it sounds like your wind instrument is giving you "station pressure" and not "baro pressure". Therefore, you would enter that into bulletflight and leave the altitude field at zero.

Did I do right?

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Dave:

I am running Windows Vista. It and Windows 7 have a built in "Snipping Tool", which allows you to save screenshots as files. If you're running one of those OS's, click on the "Start" button, and start typing in Snipping in the search box. You'll find it. It's very handy.

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

Blackwidow: I just revised my web page about barometric pressure and ballistic software. It may answer your question. I can't specifically answer it, because I have neither a Brunton instrument nor an Apple to run that program on. My wife is way too smart to consider letting me use her Ipod...

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Baro/

 

[DaveV]

Re: A comparison of ballistic programs vs actual Dope

Thanks, Lindy!!

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: One-Eyed Jack</div><div class="ubbcode-body">
PM me with your 175SMK load information, including the water volume of the fired case (Weigh the fired case with the primer in it, then fill it to the very top with water with a bulging meniscus and weigh it again. Make sure you dry off the outside of the case before weighing it with the water.) Tell me everything you know about the load and I'll run some QuickLoad calcs tonight. Also, tell me what scope and reticle you were using. </div></div>

PM sent

 

[azimutha]

Re: A comparison of ballistic programs vs actual Dope

Seems like the altitude you're firing from would have more effect because drag would initially be higher since the velocity is higher. At any rate, you have a lot more to worry about at those distances taking such a high-angle shot.

Lindy, thanks again for clarifying which settings to use in JBM (and, in turn, Ballistic). I look forward to my next visit to range to start doing more real-world comparison. The only thing I think I might tweak after that is the scope height setting since I have a 20 MOA base (I know, it shouldn't make any difference).

 

[blackwidow2912]

Re: A comparison of ballistic programs vs actual Dope

thanks lindy i think i understand now so i am doing it correctly.

 

[One-Eyed Jack]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ReaperDriver</div><div class="ubbcode-body">Will do. I'll have to wait until I get home tonight, I'm getting ready to go fly. But thanks - I'm interested to see what you find out with QL. </div></div>
ReaperDriver sent me the parameters of his load and some refined information about the range environment.

<span style="font-weight: bold"><span style="text-decoration: underline">Load Data from ReaperDriver</span></span>

* Rem 700 SPS-Tactical, 20" bbl, 12" RH twist
* Lapua brass: 57.4 grains water volume
* Fed 210 primer
* Zero conditions: 65F

* 175SMK Load
- 45.3 gr RL-15
- COAL 2.815"
- Chrono (CED M2): 2,608fps @ 10' from muzzle

* 178AMax Load
- 45.3 gr RL-15
- COAL: 2.835"
- Chrono: 2,590fps @ 10'

* 168AMax Load
- 46.4 gr RL-15
- COAL: 2.835"
- Chrono: 2,690fps @ 10'

* 1kyd actual shooting conditions
- 45F
- 4,200' ASL
- 20% RH
- Baro pressure (not station): 29.90" Hg

<span style="font-weight: bold"><span style="text-decoration: underline">Calculational Results</span></span>

<span style="font-weight: bold">Case A: 175SMK Load</span>
QL: 2,605 fps MV @ 70F
Measured: 2,608 fps @ 10'

JBM: 10.5 mils (G1)
JBM: 10.7 mils (Litz/G7)
Measured: 11.4 mils

<span style="font-weight: bold">Case B: 178AMax Load</span>
QL: 2,585 fps MV @ 70F
Measured: 2,590 fps @ 10'

JBM: 10.6 mils (G1)
JBM: 11.0 mils (Litz/G7)
Measured: 11.5 mils

<span style="font-weight: bold">Case C: 168AMax Load</span>
QL: 2,691 fps @ 70F
Measured: 2,690 fps @ 10'

JBM: 10.0 mils (G1)
JBM: 10.3 mils (Litz/G7)
Measured: 11.1 mils

The measured MV will be less than the actual MV by a few feet-per-second because the chrono was 10' from the muzzle. IMO, the QL calculations confirm the chrono data. I suggested to ReaperDriver that he check the calibration of his Falcon Menace, both the mil scale reticle and the turret clicks. If the scope calibration checks OKay, then some discussion of the wind environment and terrain might be useful.

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: One-Eyed Jack</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ReaperDriver</div><div class="ubbcode-body">Will do. I'll have to wait until I get home tonight, I'm getting ready to go fly. But thanks - I'm interested to see what you find out with QL. </div></div>
ReaperDriver sent me the parameters of his load and some refined information about the range environment.

<span style="font-weight: bold"><span style="text-decoration: underline">Load Data from ReaperDriver</span></span>

* Rem 700 SPS-Tactical, 20" bbl, 12" RH twist
* Lapua brass: 57.4 grains water volume
* Fed 210 primer
* Zero conditions: 65F

* 175SMK Load
- 45.3 gr RL-15
- COAL 2.815"
- Chrono (CED M2): 2,608fps @ 10' from muzzle

* 178AMax Load
- 45.3 gr RL-15
- COAL: 2.835"
- Chrono: 2,590fps @ 10'

* 168AMax Load
- 46.4 gr RL-15
- COAL: 2.835"
- Chrono: 2,690fps @ 10'

* 1kyd actual shooting conditions
- 45F
- 4,200' ASL
- 20% RH
- Baro pressure (not station): 29.90" Hg

<span style="font-weight: bold"><span style="text-decoration: underline">Calculational Results</span></span>

<span style="font-weight: bold">Case A: 175SMK Load</span>
QL: 2,605 fps MV @ 70F
Measured: 2,608 fps @ 10'

JBM: 10.5 mils (G1)
JBM: 10.7 mils (Litz/G7)
Measured: 11.4 mils

<span style="font-weight: bold">Case B: 178AMax Load</span>
QL: 2,585 fps MV @ 70F
Measured: 2,590 fps @ 10'

JBM: 10.6 mils (G1)
JBM: 11.0 mils (Litz/G7)
Measured: 11.5 mils

<span style="font-weight: bold">Case C: 168AMax Load</span>
QL: 2,691 fps @ 70F
Measured: 2,690 fps @ 10'

JBM: 10.0 mils (G1)
JBM: 10.3 mils (Litz/G7)
Measured: 11.1 mils

The measured MV will be less than the actual MV by a few feet-per-second because the chrono was 10' from the muzzle. IMO, the QL calculations confirm the chrono data. I suggested to ReaperDriver that he check the calibration of his Falcon Menace, both the mil scale reticle and the turret clicks. If the scope calibration checks OKay, then some discussion of the wind environment and terrain might be useful. </div></div>

Thanks again, OEJ. I can't believe how close my chrono data matched the QL results. Sweet, gives me a little bit more confidence that the CED M2 rocks.

I see QL in my future to help out with further load development....

 

[ColoWyo]

Re: A comparison of ballistic programs vs actual Dope

Reaper, let me just say that you are not the only professional pilot who was totally confused with the whole station pressure versus baro pressure. We spend our whole career using baro pressure and having it read the correct altitude when we are on the pavement and then using 29.92 going through 180. It confused the hell out of me. This thread has been very helpful to me as well.

 

[One-Eyed Jack]

Was the target really at 1,040 yards?

We've had a few theories about why ReaperDriver's range dope doesn't match the ballistic predictions. What about the possibility that the target was farther away than 1kyd?

We can use his elevation data and QL MVs to estimate the target range. When we do this, the data suggest the target was actually 20 to 60 yards farther away, with 1,040 yards the average distance.

For each of the loads here are the predicted target ranges, assuming ReaperDriver's measured elevation and the QL MVs:

<span style="font-weight: bold">175SMK Load</span>
* Assuming G1 ballistics, then target is at 1,050 yds
* Litz/G7 ballistics: 1,035 yds

<span style="font-weight: bold">178AMax Load</span>
* G1: 1,020 yds
* Litz/G7: 1,025 yds

<span style="font-weight: bold">168AMax Load</span>
* G1: 1,060 yds
* Litz/G7: 1,035 yds

The average of these six predictions suggests the target could have really been at 1,040 yards, explaining the apparently high elevations. However, the high elevations could also be explained by poor scope elevation calibration. In the event the Falcon Menace calibration is OKay, I hope RD can remeasure the distance to his target using a different technique.

 

[ReaperDriver]

Re: Was the target really at 1,040 yards?

Its quite possible that the range was off a bit. I tried to laser the steel from the shooting position but could't get a return at that range because the plate was too small I guess and the surrounding stuff was just dirt. But we were shooting from an elevated spot at the lip of a cliff and I was able to laser the edge of the cliff from the target and got 1001 yds. We might have been 5 yards back from the edge of the cliff/embankment. But not 30-40 yds.

Also my other shooters said they seemed to be fairly close to their predicted dope for 1K so Im thinking the range was close. The only way to be sure would be to get a car or a quad-runner down to the spot where we place the steel so we could laser something more reflective. But we had to hump the steel plate on foot about 200 yds off the road because it was too rough to drive right up to it. Next time, maybe we'll try to find something right near the road.

 

[308sako]

Re: Was the target really at 1,040 yards?

Deleted in protest!

D

 

[One-Eyed Jack]

Re: Was the target really at 1,040 yards?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 308sako</div><div class="ubbcode-body">Since my rifle was shooting there as well, my dope had me 1/4 MOA (sorry MIL purists) lower than called for the 1,000 yard distance. J I'll ask T and M if they had similiar issues with there dope at the match tomorrow.

D </div></div>
If you were shooting at the same time, that would also tell us it probably wasn't the wind either.

 

[aqualung]

Re: A comparison of ballistic programs vs actual Dope

Matey,

If you really are a Reaper driver, then this will make sense to you;

QNH or
QFE

My apologies to the rest of you ground-bound crawling things, it's a pilot thing.

QNH - SEA LEVEL pressure
QFE - Pressure at AIRFIELD ELEVATION

Don't know what the USAF uses, but we ALWAYS use QNH and set our minima based on that.

PHX for example is about 1300' up. Our minima for the approach would be 1500' or so, since we have QNH set.

IF we set the QFE, our minima would be set to 200'

The ACTUAL HEIGHT ABOVE THE GROUND remains the same, and the radalt tells me it's 200'. The BASELINE that we measure from makes the difference.

Hope this puts it into perspective for you.

Station pressure is QFE and the altitude you enter is ZERO. (pressure already correct for that ALT so the ballistic prog does not need to correct for alt)

The atmospheric pressure is usually corrected to sea level (read QNH). Input your altitude. (The ballistic prog CORRECTS for the alt [20' per millibar IIRC])

For you non-aircrew types, what the others above said. (Just tryin to put it into a flying context in case Reaper Driver really does drive one)

ps, Reaper driver, if you really do drive one of those baby's out of Nellis, any chance of a go??

Neil

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

QNH is the altimeter setting which will cause the altimeter to read the field altitude when the aircraft is sitting on the field.

It's usually not precisely what the sea-level pressure would be, because the altimeter responds to pressure as if the pressure existed in the standard atmosphere, which assumes a certain temperature regime. That regime may not exist - and Phoenix in the summer is a perfect example of a place where it doesn't. In the ICAO Standard Atmosphere, the temperature at 1135 feet would be about 55.03 degrees F. I've been in Phoenix in the summer. That wouldn't be the temperature...

So, QNH "fools" the altimeter into reporting that it's at 1135 feet when the aircraft is parked on the ground at Sky Harbor.

That's a distinction, though, which doesn't matter to most people. If you're a shooter, just forget that you read this...

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Paduan Learner</div><div class="ubbcode-body">Matey,

If you really are a Reaper driver, then this will make sense to you;

QNH or
QFE

My apologies to the rest of you ground-bound crawling things, it's a pilot thing.

QNH - SEA LEVEL pressure
QFE - Pressure at AIRFIELD ELEVATION

Don't know what the USAF uses, but we ALWAYS use QNH and set our minima based on that.

PHX for example is about 1300' up. Our minima for the approach would be 1500' or so, since we have QNH set.

IF we set the QFE, our minima would be set to 200'

The ACTUAL HEIGHT ABOVE THE GROUND remains the same, and the radalt tells me it's 200'. The BASELINE that we measure from makes the difference.

Hope this puts it into perspective for you.

Station pressure is QFE and the altitude you enter is ZERO. (pressure already correct for that ALT so the ballistic prog does not need to correct for alt)

The atmospheric pressure is usually corrected to sea level (read QNH). Input your altitude. (The ballistic prog CORRECTS for the alt [20' per millibar IIRC])

For you non-aircrew types, what the others above said. (Just tryin to put it into a flying context in case Reaper Driver really does drive one)

ps, Reaper driver, if you really do drive one of those baby's out of Nellis, any chance of a go??

Neil </div></div>

Thanks, I had to dust off the cobwebs of knowledge and remember that QFE was station pressure. I haven't heard QFE in years (decades) because no one uses it at all here. ut good explanation.

And yes I do drive one and I'd be glad to give you a go.... but I'm not sure where to stuff you inside the airplane since there is no cockpit in one.

 

[aqualung]

Re: A comparison of ballistic programs vs actual Dope

It's just like an EFFING big video game right??

Seriously though, I don't know how the hell you land one of those things without all of the visual and physical cues.

Neil

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">
So, QNH "fools" the altimeter into reporting that it's at 1135 feet when the aircraft is parked on the ground at Sky Harbor.

That's a distinction, though, which doesn't matter to most people. If you're a shooter, just forget that you read this...

</div></div>

Serious question.... if QNH "fools" the airplane into thinking its on the ground at 1135', why can't it also fool the bullet into knowing its at the correct alt. I know that sounds weird because the bullet will only know the airmass its in and nothing else, but if QNH is accurate enough to get an aircraft altimeter to a max of +/- 75', would it also not be accurate enough for a ballistic calc?

Maybe the difference is QNH would be fine if I was shooting on the airfield itself - but as I get further away in distance or am at an actual elev different than the airport, the interpolation starts to break down because it uses a "standard" lapse rate that is not standard in reality. Is that the issue?

 

[Lindy]

Re: A comparison of ballistic programs vs actual Dope

For shooting purposes, it's a distinction without a difference. What we want to know is the station pressure, and that we can measure.

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Paduan Learner</div><div class="ubbcode-body">It's just like an EFFING big video game right??

Seriously though, I don't know how the hell you land one of those things without all of the visual and physical cues.

Neil </div></div>
Actually, it is like playing an Effing big video game using Commodore 64 or Atari controls. The HMI is an absolute joke and I would personally like to take the engineer that designed it and the AF guy that bought it out back and shoot them in the face. The aircraft design itself is actually pretty well done, but the "cockpit" is an abortion... and even saying that is an affront to abortions.

Yes, the aircraft is very tough to land. No peripheral vision, no depth perception or "feel" or feedback in your ass as to what its doing. Its a very instrument scan intensive flight regime in the pattern. Its doable, obviously. But its way harder to land than my previous airplane or any other airplane. And crosswinds REALLY suck, even light crosswinds - because the thing is essentially a sailplane with a motor. LOOONG hi aspect wings really don't want to stop flying when its time to land.

 

[308sako]

Re: A comparison of ballistic programs vs actual Dope

Deleted in protest!

 

[308sako]

Re: Was the target really at 1,040 yards?

Deleted in protest!

 

[yotechsr]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">Actual field dope is certainly preferable to calculated dope.

People who have to make shots in conditions and/or at distances they do not have field dope for, though, may need to rely on ballistic programs.

A shooter who can "tune" a ballistic program to agree with his field results has a better chance under those conditions.

</div></div> Right on

 

[sobrbiker883]

Re: A comparison of ballistic programs vs actual Dope

ReaperDriver, you have checked the accuracy of your scope's adjustment, correct?

Sako308's projected and actual dope only being a 1/4moa off at 1K at the same place and time makes me think something is amiss elsewhere.

Lots of great stuff learned in this thread.

Thanks Lindy, as usual for sharing your AM pages! Good stuff!!

 

[ReaperDriver]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sobrbiker883</div><div class="ubbcode-body">ReaperDriver, you have checked the accuracy of your scope's adjustment, correct?

Sako308's projected and actual dope only being a 1/4moa off at 1K at the same place and time makes me think something is amiss elsewhere.

Lots of great stuff learned in this thread.

Thanks Lindy, as usual for sharing your AM pages! Good stuff!! </div></div>
No, I haven't checked the scope calibration yet. that's next on my to-do list.

And agreed, lots of great learning going on here..... Thanks for everyone's contribution.

 

[zenbiker]

Re: A comparison of ballistic programs vs actual Dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Jeff in TX</div><div class="ubbcode-body"> I read through your data, most of the responses and Low Light helped pick out some what I saw as obivious indicators for your error results.

Ballistic programs can be very helpful and some are extremely accurate, but they’re only as good as the data you feed them.

The G7 coefficient drag model seems to be all the rage these days especially with the help of Bryan Litz, myself and few others getting folks to understand the differences in coefficient drag models and getting them away from using the G1 drag model for everything. However the Sierra matchkings fit the G5 coefficient drag model much better than the G7 since it’s truly not a VLD designed bullet. How did you obtain your G7 BC? If you plucked it from a library that may be an issue also. Different rifles can shoot the same bullet differently and produce different BC’s for the same bullet. They may be close or they could be vastly different. Barrel diameter, twist rate and length are the real factors. You should try measuring your 100 yard velocity and get a good 10 round average. JBM has a quick feature that lets you plug in your muzzle and distant velocity readings and will give you your actual corrected BC. BTW, your measured 100 yard distances “MUST” be very accurate and you cannot rely on a laser range finder. If you’re off, your corrected BC will be off

Your scope height is critical to accurate results but there’s a gotcha involved with this in making your ballistic programs results and one I stumbled on by accident. If your rifle has angled bases it will throw off your ballistic program outputs somewhat. I’ve done extensive testing in the field using lots of live range data and number crunching and with my rifles I’ve found I needed to come down in my scope height by a couple few tenths of what I actually measured.

Last but not least is using barometric pressure or altitude. Pick one and never use both. Using standard readings on a ballistic program is always a receipt for inaccurate results. The most accurate results come from setting your ballistic program at 0’ altitude and using the actual measured barometric pressure. If you try to use both which a lot of ballistic program users do, your two fields with conflict and the results are skewed results.

Hope it helps and best of luck.
</div></div>

Big +1, except that I evolved using temp and altitude to determine air density, largely ignoring barometric pressure. And no program is a total substitute for trigger time, an area where I really need to improve. This whole work thing really screws with my shooting.

 

[USNIGHTGUNNER]

Re: A comparison of ballistic programs vs actual Dope

I think a program is just a good starting point

 

[para38super]

Re: A comparison of ballistic programs vs actual Dope

Tag for a good read!

 

[Pat M]

Re: A comparison of ballistic programs vs actual Dope

Save for reading