GIGO: Why Your Ballistic Calculator is Inaccurate

Sendit Ballistics, my own calculator, has a great reputation for accuracy. My algorithm takes a lot into consideration, and this results in excellent performance for medium to long range shots. However, the truth is that most all ballistic calculators will give reasonably good data.

While calculating trajectories in compressible flow (bullets flying quickly through air) requires advanced mathematics, it’s a fairly mature field. The reason your calculator gives you bad data probably has nothing to do with the calculator’s algorithm. Rather, it is a phenomenon known as GIGO: Garbage In, Garbage Out.

Your ballistic calculator relies on the following data, in order from the most important to least important:

  1. Ballistic Coefficient – This is generally a “G1” or “G7” class coefficient. All it means is that the bullet is benchmarked against a standard shaped object, and its performance is measured relative to that well known drag profile. You can get real nonsense if you input a G1 BC in place of a  G7 BC.
  2. Muzzle Velocity – Get this too wrong, and your range card will be useless past 500 meters.
  3. Weight – Inputing the wrong weight will make your bullet seem to perform better if it is too heavy, or worse if it is too light.
  4. Zero range – If you input the wrong zero range, by a large margin, you can easily get unhelpful corrections.

Less important: scope height, weather, twist rate, Coriolis effect.

Happy hunting.

Using Ballistic Calculators in the Field

Electronics versus DOPE Books

Handheld ballistic calculators are amazing gadgets that we couldn’t have envisioned at the start of this century. The ability to carry an Android, iPhone, or Kestrel device and have it provide accurate ballistic solutions is a serious advantage for a precision shooter in the field. In this article I provide shooters a simple procedure for making the most of electronic calculators in the field.

Ballistic calculators have generally supplanted DOPE books as the primary source of information for shooters compensating for bullet drop. I would urge serious precision rifle shooters to continue to collect DOPE (Data On Previous Engagements) during select shooting exercises, such as when the shooter knows atmospherics and range, and the target is small. In this way, the shooter can verify how accurate his chosen ballistic calculator is, as well as the data he has entered into it.

One way to use the ballistics calculator is to simply jam in your information, and take the shot. Of course, this approach introduces more problems: reliance on electronics, very slow correction time, and light in your face to name a few. At a minimum, you must have your rifle and load combinations pre-programmed.

An alternative way to use ballistics calculators is to use them the same way we formerly used our exhaustive collection of DOPE — to make a range card. Most shooters who are using military-style rifle setups will want to use 100 meter increments, and have their range card go out to 1000 meters or so. Notable exceptions would be police or small caliber shooters, who might want 50 or 25 meter increments, with a shorter max range.

For wind, I use a full value wind at 10mph. From this single point, it’s pretty easy to scale as needed for my 70% and 50% value winds, as well as differing wind speeds. As for elevation corrections that are in between range card ranges, just interpolate. While we know that trajectories do not change linearly, you can interpolate between the ranges as necessary and get close enough for government work.

Constructing the Card

Don’t use pen on white paper. It will become a sweaty mess, like when your mom washed your school clothes with a piece of homework in them. Rite-In-The-Rain is serviceable in a pinch. A laminated piece of paper should be the minimum acceptable range card. I wouldn’t even use that. It makes a target indicator, and isn’t as useful at night. A piece of cardboard, laminated, with luminescent tape is really cooking with oil.

Lum-tape allows the rifleman to see corrections at night.

My preferred range card is luminescent tape with a sliding window. I hide the majority of it, so that the shooter only sees two corrections through the window. This means it’s not such a huge honking light at night, and there is less of a chance of the shooter reading the wrong data entry. The rifleman can always remove the shielding for day use. The “window” is just a small cutout on one of the pieces of paper which can move up and down. See the procedure below:

  1. Brown construction paper (or spray-painted index cards), 3″ x 5″, 2X
  2. Stick-on laminate
  3. Luminescent tape, 2″ x 2.5″
  1. Construct the shielding with a pieces of laminated brown construction paper. Cut out a window in the center of the paper, about 1.5″ by 2.5″. Cut out a tab at the top (and bottom if you wish), roughly 1″ by 2″. See below:
  2. Put the lum-tape rectangle on the bottom portion of your other piece of brown paper. You can make a full-size piece, but you’ll have to cover one of the halves for the window to work right. I do this, and just cover the top half with 100mph tape. Then, I have that extra space if I need it. Don’t write your corrections just yet, wait until the end. See below:
  3. Laminate each piece of paper.
  4. Now you can write your corrections on the luminous portion with a map marker.
  5. Put the shield piece atop the range card, and move the range card up and down to put your desired corrections in the window.
  6. They aren’t connected in any way, so the shield can easily just fall off. Use the tape-on laminate to make two or three straps around the back of the shield. You’ll have to cover the sticky side on the back. Now you can simply slide the card up and down, and the two cards will stay together.
  7. See the finished product below:

There are numerous other techniques for keeping your hold-offs handy. My method is just one of them. In fact, I bet somebody in the comment section will have an idea which is simpler and better than mine. Good on them. Another good technique I know of is the tiny range card in the scope cap. Some optics manufacturers, such as Vortex, make these. You send them your DOPE, cap size, and money, and they send you a little insert.

Which Ballistics Calculator?

The science behind symmetrical projectile flight is fairly well-known by now, largely thanks to the work of the Aberdeen Proving Grounds. Those who are comfortable with calculus and compressible fluid flow should check out Robert McCoy’s seminal work on the topic Modern Exterior Ballistics.

That said, all of the calculators have slightly different trajectories. I have yet to come across a ballistics calculator that is truly terrible. Here are a few to get you started:

  1. Sendit Ballistics  – I programmed this app for iOS (Android coming soon), and I recommend it whole-heartedly. I have specifically designed this App to be “field-worthy,” featuring night vision mode, fast corrections, rapid switching between guns, and a range slider for changing targets. See the product page for more info, or download now. It’s free to 250m, and $4.99 to infinity.
  2. Hornady, Strelok, Ballistics AE – These are industry leaders, and work quite well. I believe that these Apps are wonderful calculators, but frankly over-designed for tactical shooters, and quite costly for hobbyists.
  3. Hornady online ballistics calculator – If you just want corrections to make a range card, use this. It’s free, and requires no iDevice.

Happy hunting.