NOTE: More involved questions may be covered in my
Answers to 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
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.
Can you add functionality to your programs that does...?
Possibly, it depends on what it is. Many of the features of this website were added after a user
suggested it. If you want to see it, ask.
How do I enter the angle for my scope base?
The short answer is you don't. The total angle between the bore of the rifle and the line of sight
determines the zero range. When you enter a zero range, the programs automatically calculate the
required total angle. When you zero your rifle, you take the angle of the base into account by
adjusting your sights to achieve the desired zero. The net result is that you have adjusted your
sights to a smaller angle beteen the center line of the sights and the base which is the point
of an angled based. For example, if you have a 20 MOA base on a scoped rifle and you want to
zero at 200 yards, you go the range and adjust your scope until the bullet is hitting the center
of the target at 200 yards. You then run a trajectory and notice that the elevation angle is
6.8 MOA. What does this mean? It means that when you zeroed your rifle, you adjusted the scope
until the angle was -13.2 MOA so that 20 MOA + (-13.2 MOA) = 6.8 MOA. If you didn't have the
angled base, you would have adjusted your scope to an angle of 6.8 MOA. The net result is that
you have adjusted your scope to an angle 20 MOA lower than without the base so that the total
angle (base plus scope) is the required 6.8 MOA.