Range Report Calculating Ballistic Coefficients with JBM Ballistics?

[Starbuck]

I was under the impression that in order to calculate a bullet's ballistic coefficient, one had three choices:

a) Muzzle velocity compared to and calculated against some "meaningful" downrange velocity
b) "Point of Impact" Method
c) Doppler Radar (a variation I assume of a), above)

Now, it was recently pointed out to me that the JBM "Drag and Twist" utility (here) will calculate a BC based upon a number of inputs, NONE of which include a,b, or c, above.

JBM - Calculations - Drag/Twist

Could somebody kindly 'splain this to me?

I didn't know that a BC could be calculated from the comfort of my reloading room - if in fact it can - so I want to understand this a little more....

 

[Lowlight]

That is part of the big thread that involved Bryan and Michael ...

Who's BCs are calculated without practical data vs computer predicted.

It's an approximation, and how a lot of it is done, your actual results may and usually do vary. You might as well pull the number off the box because chances are they did the same thing you are gonna do with this utility.

 

[Starbuck]

That is part of the big thread that involved Bryan and Michael ...

Who's BCs are calculated without practical data vs computer predicted.

It's an approximation, and how a lot of it is done, your actual results may and usually do vary. You might as well pull the number off the box because chances are they did the same thing you are gonna do with this utility.

Crap thought I did a search before I posted. Got a link?

 

[Lowlight]

Here you go

http://www.snipershide.com/shooting...bcs-predictions-rather-than-measurements.html

 

[damoncali]

The bottom line is that you can calculate them either with simple methods like JBM uses, or with more advanced engineering software. Neither is as accurate as *good* measurements, which have their own challenges and uncertainties. I'm not sure I believe this will always be the case, but I'm no fluid dynamics expert.

 

[Michael Courtney]

I was under the impression that in order to calculate a bullet's ballistic coefficient, one had three choices:

a) Muzzle velocity compared to and calculated against some "meaningful" downrange velocity
b) "Point of Impact" Method
c) Doppler Radar (a variation I assume of a), above)

Now, it was recently pointed out to me that the JBM "Drag and Twist" utility (here) will calculate a BC based upon a number of inputs, NONE of which include a,b, or c, above.

JBM - Calculations - Drag/Twist

Could somebody kindly 'splain this to me?

I didn't know that a BC could be calculated from the comfort of my reloading room - if in fact it can - so I want to understand this a little more....

That specific link at JBM is an implementation of Bob McCoy's McDrag method for predicting BCs and stability. Bob's paper explaining his method is here: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA098110

This method was good at its time, but is somewhat outdated, and now all the more reliable methods actually measure the BC.

Radar is sometimes used to measure BCs, but most manufacturers who actually measure their BCs use either the near chronograph and time of flight measurement of use two (or more) chronographs to determine near and far velocities. The JBM Calculators for data entry using these measurement techniques are here:

JBM - Calculations - Ballistic Coefficient (Time)

JBM - Calculations - Ballistic Coefficient (Velocity)

We've used both methods for various purposes, but most of our published BC numbers are from the near and far velocity method using two chronographs over a carefully measured distance. Even in cases where a BC is carefully measured by a manufactuter, there are some small (< 5% usually) from lot to lot of bullets from the same manufacturer and from rifle to rifle within the same lot. There can also be slight variations in BC with different twist rates. There is some advantage in measuring BCs for a given rifle and lot, especially if accurately determining the energy delivered at a given range is important. This cannot be determined from drop.

There are some subtleties when determining BC from the two chronograph method using hobbyist equipment, but most of the details are within the capabilities of long range shooters accustomed to care in the details.