## FAQ

NOTE: More involved questions may be covered in my topics section.

### Ballistics Questions

• Why can't I calculate BC from bullet drop at different ranges?
You can, but not very accurately. If you look at the bullet drop for two bullets, one with a BC of 0.5 and the other with a BC of 0.55, you'll find that the bullet drops at 300 yards with a muzzle velocity of 3000 f/s and a 100 yard zero, are -10.9" and -10.7" respectively. So if you can shoot groups less than 0.2" at 300 yards, have at it. Of course you'll still only know the BC within about 10% (the BC calculation CGI can be as accurate as 2%!).
• Why doesn't the air density change when I change the altitude?
The altitude is only used when you check "Standard conditions at Altitude". If this is not checked, the density is calculated from the Temperature, Pressure and Humidity you entered.
• Where can I get the formula for these programs?
The source code to my old CGI programs is now available under the GNU public license -- see my downloads page. I have also put the programs that I received from Bob McCoy in the downloads page of this site. The "formula" for the trajectory program is available in various sources, including the downloads page. It does involve "numerical integration" of the equations of motion (as I have implemented it...). The Sierra Reloading manual has a section on Exterior Ballistics which provides the differential equations of motion, as does "Exterior Ballistics" and "Mathematics for Exterior Ballistics". Calculation of a trajectory also requires calculation of atmospheric density as a function of temperature, pressure (sometimes humidity, if you want). I would recommend the ICAO Standard Atmosphere (see U.S. Standard Atmosphere for governing equations).
• Can I change the drag function to a better one for my bullet?
Yes and No. Ballistics Coefficient and Drag Function go hand in hand. The Drag Function specifies the "standard bullet" that the BC is comparing too. You must enter the correct Drag Function of that BC. Typically, this is the G1. There are however some manufacturers that specify different BCs using different drag functions because the force on the bullet is modeled better with the different drag function. (For instance, Berger uses the G7 for some of it's VLD bullets.) Bryan Litz' new book has many G7 BCs, measured with great accuracy. These measurements have been added to the bullet library -- just look for "(Litz)" in the description.
• How is the energy calculated?
Energy listed in the trajectory calculations is the kinetic energy of the bullet. This is discussed more in depth on the Topics - Energy page.
• Have I found a bug?
Possibly. I would recommend that you recheck your numbers on the data page and ensure you are calculating what you think you are. If you still cannot see the problem, let me know. When reporting bugs, it is important to use the error link on the output of the program. This will send me a list of all your inputs automatically.
• What are the programs written in?
All of the on-line calculations are written in C and compiled with gcc (GNU C Compiler). Most of the calculations are math intensive and require numerical integration. This (in my opinion) makes any non-compiled language unsuitable for this kind of CGI.
• Can I have a copy of your programs?
Yes and No. The old (very old) on-line version of my CGI programs have been GPL'd and are freely available for download from my "Downloads" page. The source for my new versions is for sale with certain restrictions. Contact me about purchasing the source.
• Where can I get an explanation of the terms used in your calculations?
At the top of each data page is a link to the instructions for the particular calculation. Directly under the instructions paragraph is a link to an explanation of terms used in the input and output of the particular calculation. Additionally, next to each input, there is a "?" link. Clicking on that will take you to the help page for that input.
• How important is the measurement of sight height and offset?
Not very. At longer ranges it can add a few inches of error. Measuring with a ruler is certainly accurate enough.
• Why isn't the bullet length in the bullet library?
Because I don't know the lengths of all the bullets in the library. Unfortunately, manufacturers typically only supply caliber. The only way I have to get all the bullet lengths is to buy a box of bullets for every bullet in the library, which gets very expensive with 1400+ bullets. If you measure a bullet for which I don't have a length, I will add it to the database.