How to Use a Ballistics Program - by Aaron Davidson
Aaron Davidson, Product Engineer
Long Range Shooting is the new hot item. Super magnum cartridges and all sorts of scope reticles and gadgets claim to get you there. If you know how to calculate your ballistics, you can make an educated judgment about what works and how to do it. The old method is to look up our bullet in a reloading manual, guess our velocity and estimate our drop from the ballistics chart. There are so many simple ballistics programs available, that using charts is obsolete. Excellent programs like Exbal and RSI are really affordable and offer some unique features. You just install them on your computer, and they are available anytime. Several programs are free to use anytime using your internet connection.
In this article, we are going to examine the uses and functionality of the online ballistics program available at www.thebestofthewest.net. I warn you, this may seem a little technical, but we will cover all the bases in detail—even calculating your True Ballistic Coefficient. If you get stuck, just jump over to the online forum—there are some very knowledgeable people there ready to provide you with more help if you need it. Believe me when I tell you that the first time you make a cold bore bull’s eye at a half mile, you will have forgotten about all of this painful and boring data crunching!
Trajectory Predicted by Drop Chart
Trajectory Corrected Using Ballistics Program
The basic purpose behind learning to understand a ballistics program is to help us make the shot in the field when it’s all on the line, but there are a few other reasons you may want to explore your rifles external ballistics. You might want to compare the down range performance of different bullets or cartridges, or determine a maximum range for ethical bullet performance. There are lots of claims out there, a ballistics program can help you separate hype and truth. For example you may want to know if that new super magnum you read about really performs as claimed.
As we are extending our range, ballistics charts can help us properly compensate for drop, wind, air density, and inclination. If we use the right inputs, the charts are accurate. Let’s examine the different sections so we know what inputs to use.
Entering Ballistics Data
The first thing you need to do is go online to www.thebestofthewest.net; then, click on the tab for the True Ballistics Program. With this program, if you hold your mouse over a question mark icon (?) it will have help information available. The first section we will look at is Sighting Data. The scope height needs to be measured to the nearest tenth of an inch. Measure from center bore line to the center of scope. The impact range is your near target range. If you are zeroing at 100 yards, use 100, etc. I need to rename some of these sections; we’ve had guys call in that input their far target range here. To build a range card, we want the impact height to be zero inches. If your range is not level, you can enter in the inclination angle in degrees. Leave the wind speed at 10 MPH. The last part is your click value. Most scopes have the click values labeled on the adjustment dials. Usually, scopes are 1/4 MOA or 1/8 MOA (.250 or .125), the Huskemaw Optics is 1/3 MOA (.333).
Enter in Sighting Data.
In the Load Data Section, pick your bullet from the drop down list. If it is not available, we will enter the BC value in the True BC Data section later. Enter a descriptive label for the cartridge/load description—your load data or ammo manufacturer—anything to keep it straight, for example, “7mm Rem Mag, 70.7 gr H1000, CCI 250, 3.420”, or “Long Range Elk Slayer”. The bullet weight only effects your energy calculation, if you picked your bullet from the list it fills automatically.
Load Data Section.
The next section we will fill out is the Conditions section. As your air density decreases your bullet will shoot flatter. Three variables affect air density and your effective BC: Temperature, Pressure, and Humidity. We can calculate a standard pressure for altitude if you provide Altitude in place of Pressure. You want your actual pressure measured from a barometer not a corrected station pressure. The input unit is inHg.
Conditions Section
The key pieces of information that determine the shape of your ballistic profile are: Ballistic Coefficient, Muzzle Velocity, and Air Density. We entered the Air Density information in the Conditions section. The other two pieces of information will be entered in the True BC Data section. If you selected your bullet, the Ballistic Coeff value is pre-populated and you only need to enter your Muzzle Velocity. If your bullet wasn’t in the list, you need to get your BC information off your bullet box, out of a reloading manual, or from the manufacturer’s website. I am in the middle of updating the bullet list, its 3000 lines long, and I have to be in the right mood to work on it. When you enter your velocity, the number should be entered as the direct reading from your chronograph 15’ from your muzzle. The software automatically corrects the value to the muzzle. If you don’t have access to a chronograph, estimate the velocity from a manual or spec sheet.
Cronograph
You can find the bullets BC on many manufactures boxes.
Building a Range Card
Now, the Range Card section will determine the range of values your calculated drop sheet will display. If your range is only 1000 yards, make the End Range 1000, and choose enough increments to compare data or make your adjustments in the field.
For now, we will skip the Field Shooting Data and Alternate Path Conditions sections. And we will proceed to building a Range Card. The first thing you need to do is calculate all of your inputs by selecting the calculate button in the top right hand section. This will output a drop and wind chart below that is calibrated to match your inputs. You have the option to change your output values. Some people prefer click values, some may require inches or MOA outputs, just select which units you need under the Deflection and Path headings. I like to have my charts output the range in yards, the deflection in MOA and the path in clicks. It depends on your scope and what you are trying to do.
Once the outputs are configured to your satisfaction, scroll back up to the top right hand section and print the range card. This data will provide most of the information you need to for comparison purposes, but for precision shooting, this chart will only serve to get us close to our actual drop figures. The problem arises from errors in the BC and Muzzle Velocity values that cause errors in the predicted trajectory. If the ballistics profile data on the card is going to do us any good, we need to fit or “warp” the data to match real field shooting results.
Calculating a True BC
Ok, we’ve verified that a calculated chart probably doesn’t give you the exact corrections at extreme ranges. When you are setting up a long range gun, you need to be perfect. There is no room for error. We film our hunts to air on the TV program “The Best of the West” and this places us under an extreme requirement for precise shooting, we don’t just need to kill, we need to drop ‘em in their tracks. This extra “True BC” step guarantees that your elevation corrections will be spot on, for precise and ethical shot placement. What we will do is use the range card to shoot two data points in the field. The first is close, say 100 yards. The second needs to be as far as you want accurate data. We shoot the second target anywhere from 800 to 1200 yards.
When we shoot the Field Shooting Data, we will record Muzzle Velocity if we have a chronograph, Temperature in Fahrenheit, and Barometric Pressure in inches of mercury (inHg), you can substitute Altitude in feet for pressure. Then we shoot a target at the impact range and zero the gun for point of impact. Now, using the range card we dial the proper MOA or clicks into the scope to compensate for our far target drop. If it is pretty close, we can just measure how low or high the group center is above the target center. Otherwise, just add or subtract a few clicks to get it closer or right on. The information we are recording is the exact Far Target Range, the number of clicks dialed in from zero, and the number of inches the group center is below (-) or above the target. Be sure to include the (-) sign if the group is below.
Armed with this information, we will go back online to the True Ballistics Program at www.thebestofthewest.net and re-enter the Sighting Data, Load Data, Conditions, and True BC Data sections, using any new information recorded in the field. Then we will enter the Field Shooting Data that we recorded during our shooting session.
The last step is to decide if you want to “warp” your BC or your velocity to make the data fit. Just select the bubble to the left of the Ballistic Coeff or Muzzle Velocity inputs in the True BC section, and then click on the Calculate button. The BC or Velocity number will be automatically updated, and a new drop chart will be output. Now check these drops against field data to ensure accurate True BC data.
If you are building a BDC turret, this True BC and Velocity are the numbers you need along with your average Altitude and Temperature of your hunting conditions. Now your calculated trajectory will match your actual shooting trajectory and you will know the exact corrections for drop and wind at all ranges. By examining the velocity numbers, you can determine the range at which you bullet drops below 2000 fps (my recommended minimum hunting velocity). You will know which bullets and cartridges retain more velocity for extended terminal performance. You will be more informed and more prepared, and ready to make that shot for that once in a lifetime opportunity. Remember, cold bore half mile bull’s eye—it’s worth the effort! Be safe, and collect good data, next time we’ll look at using this data with different methods of compensation: MOA dials, BDC turrets, and hold over reticles.