F T/R Competition Is JBM lying to me?

[9sigman45]

During the LongRange Regional match at Butner this weekend I was suprised to find that at the end of each day my elevation was at 27.75 MOA. I have a solid 200 yard zero and was shooting 185gr Berger LR BTs atop Varget (i.e. supposedly temp insensitive) It was a very hot weekend, topping out in the mid 90's both days. Both mornings I started out at 28.5, but by the end of the day would have come down 3/4 MOA. I know my pressures were reaching their limits as I blew 1 primer in the last match each day (never had this happen before).

I've always found the JBM calculator to be pretty darn close, but working the data backwards to determine the velocity JBM says I was pushing the 185 @ 2,865 fps. WOW! I didn't think that was possible, and it certianly wasn'y my intention.

Is JBM correct? Or is my method of backing into velocity flawed?

 

[spiritzerothree]

Re: Is JBM lying to me?

Did you have good environmental data put in? You'd be surprised what a change in temp/humidity/barometric pressure will do at 1000 yards.

 

[Chanonry]

Re: Is JBM lying to me?

Are you using live atmospheric data? Temp increase and your drop decreasing is consistent. If you model this change over your range what does JBM predict?

You have not provided any data to allow us to perform the analysis.

Also how are you modelling bc? Assuming you are shooting a 308 then mv looks high again suggesting an error modelling atmospherics and/or bc at range.

Close range errors= tweak mv, long range =tweak bc. I suspect it is more fundamental than that.

Previous analyses may have been close because conditions were close to those modelled and/or other errors cancelled out. A substantial change in atmospherics at long range will lead to noticeable deviations from 'standard' models.

 

[Trigger Monkey]

Re: Is JBM lying to me?

I had to dig a little bit through your posts but the best I can figure is that you're pushing the Berger 185 BT at about 2760 fps. Using this is and some data from Weather Underground for the Durham area this past weekend I was able to fool around a little with JBM. Starting out I used the 2760 as a baseline velocity with a sight height of 1.75". For environmetnal data I plugged 100' ASL, 84*F, 50% RH, 30.09 inHg into a DA calculator and came out pretty close to 2000' DA so I rounded up. Plugging figures into JBM that will equal 2000' DA I came up with 1000 yard dope that was consistent with what you were saying, which was about 28.5 MOA. Using conditions for later in the day (95*F, 30.00 inHg, 24% RH) I plugged them directly into JBM and only adjusted the velocity slightly to 2795 fps. The data matched up perfectly to what you saw as JBM spit out 27.8 MOA to 1000 using the 200 yard zero.

So it looks like it was a combination of changing environmental conditions and temperatures uping the velocity on the ammunition a little.

 

[9sigman45]

Re: Is JBM lying to me?

Sure, I provided "some" data (location, temp, bullet <i.e. bc>, zero, distance and drop).

I'd have to research the the humidity and pressure as I didn't have that available to me while at the range. As for the conditions I only entered the temp and altitude before running the calculation. I have done this in the past with good success. I realize I did a "not so exact" calculation, but I'm only looking for a general verification. I'm happy with the elevation POI variation achieved, and I'm not bitching about a percieved JBM deficientcy, I'm just shocked at the calculated speeds.

 

[9sigman45]

Re: Is JBM lying to me?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Trigger Monkey</div><div class="ubbcode-body">I had to dig a little bit through your posts but the best I can figure is that you're pushing the Berger 185 BT at about 2760 fps. Using this is and some data from Weather Underground for the Durham area this past weekend I was able to fool around a little with JBM. Starting out I used the 2760 as a baseline velocity with a sight height of 1.75". For environmetnal data I plugged 100' ASL, 84*F, 50% RH, 30.09 inHg into a DA calculator and came out pretty close to 2000' DA so I rounded up. Plugging figures into JBM that will equal 2000' DA I came up with 1000 yard dope that was consistent with what you were saying, which was about 28.5 MOA. Using conditions for later in the day (95*F, 30.00 inHg, 24% RH) I plugged them directly into JBM and only adjusted the velocity slightly to 2795 fps. The data matched up perfectly to what you saw as JBM spit out 27.8 MOA to 1000 using the 200 yard zero.



So it looks like it was a combination of changing environmental conditions and temperatures uping the velocity on the ammunition a little. </div></div>

See, I knew there was some one out there that knew alot more about these online calculators than I do. Thanks for the feedback, that's much more inline of where I was expecting my velocity to be. A 100 fps gain was blowing my mind, so I knew my method was flawed.
Thanks for the help TM.

 

[Greg Langelius *]

Re: Is JBM lying to me?

I use JBM, and as a long time computer programmer, I can say two things with some certainty.

JBM is a very reliable program (which is why I use it).

No program's result will be any more accurate than its input.

Interpreting the above, when JBM and reality don't jibe; it usually means I'm doing something wrong. Time and time again, it turns out I've gotten my environmental factors wrong.

Winds along the trajectory are key factors, and are also difficult to guage accurately.

For this reason, I think it pays to do what I call 'passive outcome observation'.

Where unlimited sighters are permitted, I use all the time allotted. Then, once I have a basic zero determined, I wait for distinctly different wind flag attitudes and fire a round for each, with no compensation from the basic zero setting and hold.

I do this just to see how deflection is influenced by the altered condition. This allows some insight into employing additional sets of conditions, and allows less waiting on conditions.

Additionally, scope height above the boreline needs to be precise.

IMHO, humidity makes a difference. But generally, for 1Kyd and inward; it's not usually enough to emerge from the background error margin.

Greg

 

[sticshooter]

Re: Is JBM lying to me?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Trigger Monkey</div><div class="ubbcode-body">I had to dig a little bit through your posts but the best I can figure is that you're pushing the Berger 185 BT at about 2760 fps. Using this is and some data from Weather Underground for the Durham area this past weekend I was able to fool around a little with JBM. Starting out I used the 2760 as a baseline velocity with a sight height of 1.75". For environmetnal data I plugged 100' ASL, 84*F, 50% RH, 30.09 inHg into a DA calculator and came out pretty close to 2000' DA so I rounded up. Plugging figures into JBM that will equal 2000' DA I came up with 1000 yard dope that was consistent with what you were saying, which was about 28.5 MOA. Using conditions for later in the day (95*F, 30.00 inHg, 24% RH) I plugged them directly into JBM and only adjusted the velocity slightly to 2795 fps. The data matched up perfectly to what you saw as JBM spit out 27.8 MOA to 1000 using the 200 yard zero.

So it looks like it was a combination of changing environmental conditions and temperatures uping the velocity on the ammunition a little. </div></div>ok,so are you saying we should run are current conditions through a DA calculator before using jdm.if so,do we plug in the DA elevation instead of our actual MSL altitude.i love learning.

 

[Trigger Monkey]

Re: Is JBM lying to me?

You can do it like that after the fact to figure out what the DA was at a certain time but I was just doing to illustrate how the environmental conditions could've changed the trajectory in this particular instance.

Typically I use a DA calculator, This One Actually, and play with the temperature and barometric pressure only until I get the DA figure that I want. For example if I use 70 and 28.8 inHg in the calculator it will give me about 2000' DA. Now when I go to JBM to figure a dope chart for 2000' DA I only input 70*F and 28.8 inHg in the conditions box. Now if I wanted to adjust the DA I would just play with the barometric pressure in the calculator to get the desired DA figure. I've been doing it that way for quite a while and it's worked out pretty good so far but I've been finding myself using the FDAC more and more.

 

[XTR]

Re: Is JBM lying to me?

I use my Kestrel to input DA and I've found that usually makes the output w/i a click or two of what I've sen in the field.