Ballistic Coefficient Calculator

[carnaby]

I originally posted this in the reloading section but got no feedback or interest there, too many pistol shooters. This is a better sub-forum for my post.

I created a Universal Windows App that takes bullet caliber and mass, muzzle velocity, environmental data, scope height, and drop data, and calculates the ballistic coefficient of your bullet. The cool thing about this app is that it computes G1, G2, G5, G6 G7, and G8 BC for your bullet, and ranks them according to how well the work for your projectile. The FOM (figure of merit) is a metric used to rank the different models against your projectile. It's not the best yet, but can be improved in the future.

This app will work on any Windows 10 device, including desktop, laptop, tablet, and phone. I've put it out for free for the first month so I can get some feedback. It's an early Beta version so the UI is underwhelming and the app may have some bugs, but I want to see if it's useful for target shooters and hand loaders.

On your Windows 10 device just search isochronX in the store and you should see it there.

Anyway, the app requires muzzle velocity, scope height (axis of the scope above the axis of the bore), zero range (typically 100 yards) and then drop data at any interval down range. Drop is positive down, and is measured in inches. Range defaults to increments of 100 yards, but can be changed.

There are a lot of data fields that aren't used, like bullet brand and model, and scope Moa and Mrad per click, but some data is critical, including muzzle velocity, sight height, caliber, weight, temperature, pressure, and relative humidity (relative humidity doesn't have that big an effect so the default is fine if you don't have a way to get this data).

The app will work with 100 yard zero and one additional data point, i.e. 200 yards, but of course gets better with more data points. I recommend that you use group average drop over 3-5 shots for distance beyond 200 yards, especially if your rifle is less than MOA with a given load.

I want to see if it might be useful and warrant further features and improvements. It's a lot of work

 

[MCMXI]

Typically the BC of a bullet is determined empirically by measuring the velocity at different ranges from the muzzle. Accounting for atmospheric conditions, the BC is backed out and the more data points the better the estimated value. Since you use bullet drop as one of your inputs, which I assume is to estimate velocity, how do you avoid the chicken and the egg issue?

 

[carnaby]

MCMXI, that's the secret sauce. You can try it against a ballistics calculator in which you assume a known BC and all the environmental data etc, and you'll see that this app comes up with the BC that you used to get the drop data. Ballistica or any other point-mass based calculator will be best for comparison.

 

[TRX]

Mann did it by putting a series of paper screens out at known distances and measuring the drop from point of aim.

Those tests told him many things, including that drag is not proportional to velocity.

 

[carnaby]

TRX, what this app does is similar, and it uses point mass method for ballistics. Works really well. For one, it shows that Lapua has done their homework and their published G1 and G7 for their Scenar bullets are very accurate. I've tested against these.

 

[briansmithwins]

Mann did it by putting a series of paper screens out at known distances and measuring the drop from point of aim.

Those tests told him many things, including that drag is not proportional to velocity.

Not any kind of aeronautical engineer, but supersonic drag behaves very differently than subsonic drag.

BSW

 

[carnaby]

briansmithtwins, this is true. And the BC varies over Mach number, so that makes it even more challenging. However, an average BC over a given Mach number range typically works pretty well. Also note that the math behind the app is partly based on the information in the textbook "Modern Exterior Ballistics" by Robert McCoy.