Range Report JBM Calculation newbie help

[skatz11]

I'm looking for some advice on using the JBM calculations to get me started with some dope for my new AR SPR setup. I just want a good guide to get me started once I get out to the range. It seems the biggest problem is user error when it comes to the inputs. What inputs will have the most significant effect on the outputs?

 

[runngun]

Re: JBM Calculation newbie help

I would think that velocity and environmental conditions would be the most likely place to have error if the bullet BC is in the database. If the bullet you are using is not in the database, then the correct BC could be an additional potential error.

Also, there could be perceived error if your knobs are not accurate (but this is NOT an error due to user input).

 

[Lindy]

Re: JBM Calculation newbie help

This is about the minimum data set you must enter for the program to work:

Ballistic coefficient
Bullet weight
Caliber

Muzzle velocity

Sight height

Zero range

Temperature

Pressure - this is the tricky one. See the section on entering atmospheric parameters in this link:

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

Humidity - however, if you checked the box which says, "Standard Atmosphere at Altitude", that box will not matter. That's one of the reasons why it's better to have your own pressure reading, because then you can also enter the humidity and it will calculate that.

Down at the bottom, check the boxes which say:

Elevation correction for zero range
Target relative drops

 

[azimutha]

Re: JBM Calculation newbie help

The hardest part to understand is the atmospheric parameters and I credit Lindy for helping me confirm the correct way to enter them. The main thing is:

1) If you are using a pressure derived from a weather forecast or similar, then that is a corrected pressure and you need to enter the actual altitude as well as check the "Pressure is Corrected" checkbox.

2) If you are using station pressure, either set altitude to zero or uncheck "Pressure is Corrected"

As an example, use 29.92, 6000 ft. and "Pressure is Corrected". Then try it with 23.98 and zero out the altitude. Then put anything you want into the altitude field and uncheck "Pressure is Corrected". All three variations will give you the same results.

 

[skatz11]

Re: JBM Calculation newbie help

Thanks for the help! That was just what I was looking for.

 

[yotechsr]

Re: JBM Calculation newbie help

Regarding velocity question. I have been tinkering a little on this JBM site as well. I dont have a chronograph or access to one to get a precise measurement of velocities. I have the stock rem sps tac 20' Anyone compiled any data on black hills smk 175's, fgmm 168's and hornady 110' from this weapon? BC numbers would help also. From what I have read (old tac ops study) I am losing a few hundred fps with a shorter barrel.