Simple ballistics hack

This is a very, very simple C implementation of a very low-tech ballistics calculator.

I first wrote this program in 1994 when I started shooting high-power rifle competitions, and was sick of having to boot DOS to run the free ballistics calculator I had.

The method I implemented here is ancient: the Siacci method. It uses tabulated ballistics data that is scaled for the ballistic coefficient used. I have tables here for G1 and G7 ballistic coefficients. The tables are interpolated to obtain time-of-flight to various distances and from that bullet drop.

This is NOT a high-tech, modern ballistics calculator and would be fairly useless for precision shooting solutions.

It is, however, plenty close enough for getting your initial guess at sight settings for various differences given a good zero and good measured muzzle velocities.

For best results, get good chronograph readings of your loads and record the temperature and altitude at which you obtained them. The program will take optional arguments for the temperature and altitude at which you want the charts printed.

The main program, "simple"

The main program produced by the makefile is called "simple" (I was very imaginative when I came up with this), and it takes command line arguments:

• -a followed by altitude in feet
• -p followed pressure in inHg
• -t followed by temperature in Farenheit
• -v followed by muzzle velocity in feet per second
• -c followed by ballistic coefficient
• -r followed by max range in yards
• -n followed by integer number of distances to compute
• -h followed by height of the sight over bore (in inches)
• -w followed by windspeed in MPH
• -z followed by range in yards at which rifle was zeroed
• -7 selects the G7 ballistics table instead of the default G1
• -T followed by a quoted string sets a title for the load

The "loads" file contains example command lines some of my favorite loads.

an alternate program, "abt"

This stood for "awkable ballistic table", because back in 1994 I was doing a lot of work writing simulation programs that had voluminous output, and used to use formatting of special lines in that output that could be picked out by the Unix utility "awk" for plotting.

abt was originally just a copy of simple with all the output simplified. They have diverged, because I basically never use abt anymore, and kept working on simple a little bit.

On accuracy

While this is NOT suitable for computing precision shooting solutions, I have compared its output to many other higher-precision ballistics calculators that are available for free or for cheap all over the place nowadays (like, on your phone). I have found that the solutions computed by this program for elevation and windage are usually within a half to a quarter minute of angle of what the higher precision (and much better) programs get.

So for a quick-and-dirty table to print out before heading out to the range, simple will do the job. Don't try to win any competitions by expecting these tables to be perfect --- but they will be good enough to let you get close so you can make up real dope tables with a minimum number of shots.

History

As I said, I wrote this in 1994 when I started trying to get serious about high-power rifle competition, and only had access to a 200 yard range on a regular basis. I needed to be able to compute sight adjustments for 300 and 600 yard shots before I headed out to competitions, and this exercise got me there.

This stuff was originally written in K&R C, not ANSI C. I updated it in 2013 to what is roughly its current form. Why? Because between 1998 and 2013 I had stopped shooting high-power competitions, and in 2013 I started up again. I wanted this program to work again.

I have occasionally considered rewriting this program to be less cryptic and more modern, but why bother? The method it implements dates back to the 19th century, so why shouldn't the code look ancient, too?

There is the beginnings of a LaTeX explanation of how this program works, but I abandoned it almost as soon as I started it. Don't even bother building it, it's only a few paragraphs and doesn't explain anything. If you really want to understand what it's doing, any book on exterior ballistics should explain the Siacci method (probably right before they dismiss it as inadequate for modern needs and explain more modern methods). You can start your investigating on Wikipedia

Oh, and in drop.c I make reference to "McShane, Kelly, and Reno". This is a reference to the book "Exterior Ballistics" which is long out of print, but may be found in digital,google-digitized form right here.

Example usage and output

A ballistics table for what is basically XM193 (though my personal handload) would be obtained with the following command line:

simple -a 5200 -t 65 -v 3100 -c .243 -r 600 -n 6 -h 2.625 -w 10 -z 200 -T ".223 55 gr load"

This specifies that we're requesting a ballistics table for an altitude of 5200 feet above sea level (where the range I'm using is located) at 65 degrees Fahrenheit. The range extends to 600 yards and we're asking for 6 intermediate ranges (so it'll be every 100 yards). The rifle is zeroed at 200 yards and the wind is blowing at 10 miles per hour. The sights on the rifle are 2.625 inches above bore line. The cartridge uses bullets with ballistic coefficient 0.243, and the muzzle velocity is 3100 feet per second.

The output will be:

.223 55 gr load
Drag function: G1 Drag Function
Muzzle velocity=3100.0 f.p.s.
ballistic coefficient at STP=0.243, corrected to=0.297 
range=600.0 yds., rifle zeroed at 200.0 yds
altitude: 5200.00     Temperature: 65.00    pressure: 24.47
Wind velocity 10.000000 MPH (176.000000 InPS)
 Drop from bore line at zero range 8.380592 inches, 3.990758 MOA

Range          Vel           Y(R) (MOA)             T(R)         defl(R) (MOA)
------        ------        --------------         ------        -------------
    0         3100.0         -2.62 (  0.0)          0.000           0.0 ( 0.0)
  100         2776.1          0.94 (  0.9)          0.102           1.0 ( 0.9)
  200         2474.4          0.00 (  0.0)          0.217           4.1 ( 1.9)
  300         2192.2         -6.62 ( -2.1)          0.346           9.7 ( 3.1)
  400         1928.4        -20.49 ( -4.9)          0.491          18.4 ( 4.4)
  500         1687.5        -43.80 ( -8.3)          0.658          30.6 ( 5.8)
  600         1470.6        -79.49 (-12.6)          0.848          47.1 ( 7.5)

Note that bullet drop ("Y(R)") is given in inches and in MOA (in parentheses), as is windage ("defl(R)"). "Vel" is the velocity of the projectile at the tabulated range, and "T(R)" is the time of flight (in seconds) to that range.