Understanding your Ballistic Computer

Ballistic Software is just a starting point, but all things equal, you should be within at least 1 MOA of most ranges. It might vary a little, but overall it should be very close. The computer cannot for all the variables and we don’t tell it everything it really needs, which is why we true.

UNDERSTANDING YOUR BALLISTIC COMPUTERS

To start off, a ballistic computer is not a magical device. These applications are not conducting complex calculations but in reality they are just inserting averages into the mathematical formula so the solution provided will give the end user a solid starting point towards hitting the target. It’s designed to get you close, not necessarily perfect as the computer has no way of understanding all the variables that go into shooting our rifles. It’s missing a lot of external data, and your PDA or Smartphone is not capable of executing the highly complex calculations necessary to actually give you a guaranteed hit. This requires we do our part in order to help it along.

Remember the computer cannot make up for any variations the shooter adds to the shot. Shooter error cannot be seen or accounted for until after you already made the shot. We need to acknowledge up front we are the biggest variable when it comes to hitting the target.

If you start off using your ballistic computer and find you are within 1 MOA of your POA, look at this as an excellent starting point. You cannot expect it to be any better than you can group at the given distance. If the computer tells you to use 12 MOA to hit a target and you only need 11.5 this is a total win.

STEP ONE – CALIBRATE YOUR SCOPE

The very first thing you need to do before you can expect any realistic results is to calibrate your scope. Small errors within this mechanical device will get compounded over time, or in our case, distance. Consider the difference between adjustments that are based on True MOA vs an optic that is using Shooter MOA or IPHY. (Inches per 100 yards). If we look at True MOA as 1.047” and one that is rounded of to 1” per hundred yards, you can have 20” of variation at 1000 yards. This is huge, so know what you have, test it, then calibrate the software to match. Most software allows you to change your click value. Get out at a verified distance and measure the adjustments / click value before you start. Don’t take it for granted the range you using has targets at 100 yards. I use a 300ft tape measure to verify.

You can do this one of several ways, but one easy way is to use a 4ft Level. Start with the reticle at the very top of the level which is placed at 100 yards. Then dial UP your elevation until the reticle is touching the very bottom of the level. (you can use a simple yard stick too) Once the reticle has traveled 48 Inches, covert the number of clicks it took to move that distance. This will give you the actual click value for your scope. You can do with both Mils & MOA.

Back in the day we used to a Barber’s Pole which had black and white striped painted on the pole to help like up the reticle as well as calibrate the scope. The stripes were painted 3.6” apart so you can easily check reticle sub tension and test your adjustments.

STEP TWO – WHICH BC TO USE G1 OR G7 ?

You need to add a bullet to your computer. So this means setting the weight and BC. Choose this from the library or manually input the values.

Does it matter which BC Drag Model you use: No, providing you use the correct number for that model. You cannot use a G1 value and tell the software you want to calculate that number using the G7 Drag Model. The numbers are different so understanding which one you are using is important. Some software will let you choose one value from it’s library then allow you pick a different model. Do not confuse this part. If the number is greater than .3 odds are you have a G1 Drag Model. If the number is below .3 you are probably using G7.

It is true that Point Mass is better suited to using a G7, however we have seen G1 values to be every bit as accurate because we have a long history with it. Point Mass is used in most programs, except ColdBore and Field Firing Solutions which is a Pesja Based solution. Pesja based solutions are not drag dependent like P.M. With FFS there is no option to use G7 as it’s unnecessary to get an accurate solution.

Manufacturers values are overly optimistic and are given at a maximum velocity. In some cases the BC listed by the manufacturer is for a Muzzle Velocity your average rifle will not reach. Usually between 2800fps and 3000fps so if you are going slower, consider the Manufacturer’s listing a marketing number and not an actual representation of the BC you will use. So that means when it comes time to true your software you will first adjust the BC a bit. Small changes go a long way here. And yes it is a bit of trial and error unless you have two chronographs to actually measure it. This is the best way to find out what your rifle is doing but not many have access to 2 chronographs.

Bryan Litz’s G7 BC is based off a better average velocity, however understand we have modified our understanding of the most common BCs so we don’t always default to the manufacturers listing. A prime example of this is the 175gr SMK. Sierra gives us a BC of .505, however we tend to use .496 as a starting point. We know we are not shooting a 175gr at 2800fps so we dont’ use that value. Both G1 and G7 have been around for more than 50 years. It is only now that we are establishing a G7 value because of how the programers are using Point Mass when it comes to solving. They are using a pure form of the Drag Model so it leans towards G7 a bit more. But since we are using averages any way, both work equally well so do not be intimidated by this. If G1 works better, and it might, use it.

BANDED OR STEPPED BCS.

This is actually the best way to use a ballistic computer if the software is giving you this option. You can easily look at this by going to JBM online. If you pick a Sierra 175gr MatchKing from the library and do not choose the (Litz) version, JBM will automatically Band or Step your G1 BCs for you. The values are readily available there. So what does this mean to Band or Step your BC.

When we talk about wind we always ask, Wind at the Shooter, Mid Range or at the Target. This is very common. So think of banded or stepped BCs as being a value at the Muzzle, Mid Range and at the Target. Same thing.

We know BCs are based on not only the bullet design but the velocity. We also know that since the bullet is constantly falling as well as slowing down that value is gonna change. So want a Banded BC does is, tells the software to adjust the drag curve based on the range and muzzle velocity. We have all see the outputs given to us by JBM. It includes Time of Flight as well as the projected Muzzle Velocity at a given range. Where most use a single BC number that is asking the software to apply that to the entire flight path of the bullet, the Banded BC corrects the path.

you can do it for both G7 and G1, as I suspect you’ll start seeing more software work this way. But for now, there are several programs that automatically band just G1. A banded G1 will certainly be more accurate in more cases than single average from either G1 or G7. So you might want to experiment with banding numbers.

STEP FOUR – MUZZLE VELOCITY

You have to have an actual muzzle velocity for your computer to work. You can reverse engineer the number based off your drop. However if you are serious about Precision Rifle shooting you’ll invest in a chronograph. In my local Precision Rifle Class we Chronograph every student’ s rifle before we go out to the range. It’s one of the most basic necessities to use a ballistic computer.

Now, understand, not all chronographs are perfect, so this is another place where we find a lot of errors. Never mind the fact many people guess as to what their MV should be, that is bad enough. Once we move to true your ballistic software you’ll also adjust your Muzzle Velocity to match. We’ll just assume it’s an error in the chronograph, all things being equal.

With a chronograph, the more rounds you put over it, the better your average will be. When I am mounting a new scope or trying out a new rifle, I will chronograph it as much as possible. The more data points you have the better. With any load there is a Standard Deviation, and Extreme Spread so in reality we have no idea the exact MV until after the shot has been fired. This is why people strive for single digit ES / SD numbers. The smaller your variable the closer to center you’ll be.

If you don’t have a chronograph, expect to conduct a bit of trial and error. You have to shoot your rifle to distance, record your data and then reverse engineer the values until the software matches your actual dope. Which is just a long way of saying you’ll true your software because that is what you are doing. Some software like FFS has a utility to help with this as well as help calibrate your scope.

STEP FIVE – SIGHT HEIGHT

The computer needs to know the distance of your scope over the height of the bore. This is measured from the center of the scope tube to the center of the bore and usually done around the area of the receiver. Many will have the gas port on the receiver or you can bolt, etc. The value of this should be within an 1/8 of inch at the most. If you can get within a 1/16th you are good, but bordering on going too far.

The default for this is usually 1.5” however most tactical rifles and scopes are around 2.0. Understand the cant in your base has no bearing on this calculation. It’s just the physical distance between the center of the scope and the center of the bore. Measure it and move on.

STEP SIX – WEATHER CONDITIONS

There is a lot of confusion where with weather conditions. The main point of confusion is around the Barometric Pressure and Altitude. Mixing these values up can have a big effect on your end results. An error here can tell the computer you are at 8000 ft above sea vs 4000 ft.

BAROMETRIC PRESSURE VS ABSOLUTE PRESSURE OR STATION PRESSURE

Barometric Pressure
Weather Underground

Barometric Pressure is used with Altitude and it is the corrected pressure reading the man on the news gives you no matter where you are in country. If you do not live at or close or sea level and you are using 29.92 or something similar that means you are barometric pressure. If you allow the Smartphone to pull the weather for you, it is using Barometric Pressure and will give you an altitude reading as well.

Nothing wrong with using it. However…

Station Pressure or Absolute Pressure is based off of where you are standing. Station Pressure is a function of altitude. For every 1000 ft of elevation you go up above sea level you will lose 1” of pressure. So sea level is represented by 29.92, in Denver at 5280, you have a Station Pressure of 24.92 because you have lost 5” of pressure. Ballistic Computers like this number as it’s a better representation of what the bullet thinks.

if you have a followed the threads on SH, you would have seen the people from Kestrel tell us that the Horus & Applied Ballistic software built into the Kestrel units are pulling this value directly from the Kestrel. It skips the other stuff and just uses Station Pressure.

With most software if you tell it you are using Station or Absolute Pressure it will gray out the altitude and prevent you from adding that value in. If you combine altitude and station pressure the software will think you are twice as high as you are.

As may have noted, changes in temperature will have a bigger effect so keep on that value as you shoot throughout the day.

If you can help it, do not use the information being pulled down from the smartphone. They can pull that data from a variety of places, none of which are going to be where you are shooting. I have seen some Smartphones pull data as far as 60 miles from the range. Will it get you close, sure, but let’s not use it if we can help it.

If you have not set up the App to know the conditions when you zeroed the rifle, turn that off and just use Current Conditions. If you are starting from scratch or traveling, then use both.

If you are setting up the rifle and software in Colorado and will be traveling to California it’s best to enable both zero conditions and current. This will help adjust the software for the change. If you can re-zero in California prior then you can just use Current Conditions.

DENSITY ALTITUDE

Density Altitude
Density Altitude

I do not use density altitude with my software. Density Altitude readings are best used under field conditions and are not as precise as using the actual weather conditions. If you have a chart and are going battery free when in the field, DA is an excellent way to solve the problem. However with software it was more of less an after thought. The software likes real values so use numbers. If you understand this and still want to use DA because you are not too worried about sub MOA results, then go for it. But if you are going through the trouble of using the software give it the best information you can.

OTHER OPTIONS

Ballistic Computer
ColdBore Ballistic Computer

My advice to any one using these “Apps” especially for the first few times. Turn all the other options off, like Spin Drift, Coriolis even Powder Temp. Unless you have determined what the powder temp variation actually is, do not use it.

Remember many of these options are flat rate values, and not actual calculations.

SHOOTER DRIFT

There are three things we have to be concerned with when shooting,

Range – Wind – and our Fundamentals of Marksmanship.

After that, it all tends to be noise until we have reached an elite point in our shooting. If you don’t consider yourself in the Top 10% of the game, odds are you are not going to be able to exploit the values of these other variables.

Shooter Drift accounts for a lot of what we see that cannot easily be explained away. Back in the day, the very first ballistic software sold commercially was ExBal. Gerald Perry used “Shooter’s Drift” as a way to account for both the shooter and things like SD. He required you picked the best day possible, then shot at 600 yards to determine any right or left drift you observed. Then you added that offset to his “Shooter’s Drift” dialog to account for it moving forward.

A right handed shooter tends to pull the shot right. So when adding in things like SD which gives you a left hand correction, you can fix any error we put into the shot. This works the opposite for a left handed shooter.

So if you can see a change in your accuracy by using SD and turning on the feature, use it. There is nothing wrong with doing that. However understand these are flat rate numbers. You need a 6 DOF computer to calculate true SD. Our Smartphone is not doing this. A little test I use is to look at the solution for SD at 1000 yards using a 100 yard zero. Then change the zero range to 600 yards and see the difference. Most will show you the same value because they just insert 1 MOA for SD regardless of the variables. 1 MOA is easy to hide or help for the average shooter.

Coriolis is only going to work if you shoot an azimuth to the target and know where you are standing in the world. So unless you do this for your range, don’t worry about it.

You can experiment with all of this, but first see how things work without using any of it. I find it works better.

These programs are just a starting point, and designed to assist the shooter. They are not there to replace the shooter. So understand we put just as much into the shoot, and the computer cannot fix that. The better pieces of software have ways of helping us adjust for it, and predict it, but we still have to put in the work and record our data.

HOW DO YOU US THE KESTREL WITH A BALLISTIC COMPUTER

This seems to be a hot button issue this, especially with the Sniper’s Hide Cup Fast Approaching.

As we discussed in the ballistic computer lesson, the kestrel is a valuable tool for the precision shooter, however a lot of people seem to be confused by it’s use. In the video you can see the settings so you can match them up to your location, I have also included the corrected pressure data so you can see how both will work.

STATION PRESSURE OR ABSOLUTE PRESSURE

Depending on what software you are using, the computer may ask for Station Pressure or Absolute Pressure. (iPhone programs have an Absolute Setting) In order to get this reading from your weather meter, you want it kept at the factory settings. This means:

In the Barometric Pressure Screen, the Reference Altitude should read: 0 (Zero)

Then, in the Altitude Screen, the Reference Barometric Pressure should read,

29.92 (Sea Level)

It doesn’t matter where you live, these are the settings you want. No understand with a Kestrel that includes a ballistic solver, the software will grab this information regardless of how you set it. By Default it uses Station Pressure.

BAROMETRIC PRESSURE

For shooting, altitude is a function of barometric pressure. If you do not have access to a kestrel you can use what the weather man gives you, or what a computer will say. If you use this information, you have to tell the software where you are elevation wise, because this is corrected pressure. Example below is a grab from Weather Underground. (an excellent resource by the way)

As you see above, it matches my video pretty close in terms of corrected pressure readings. This is taking into account the altitude of the location being reported on, so the Barometric Pressure is corrected. If you notice the amount of information provided, this is very accurate and worth using. I reference Weather Underground for my data all the time.

When you use 29.92 or any pressure value from the news, or online, you have to include the altitude. With more software today this is the only way you can put the altitude in because if you check Station or Absolute the altitude dialog box will be grayed out and not working. They have done this so you don’t compound your weather. If you used Station Pressure and Altitude together you are doubling where the computer thinks you are. 5,000 ft is now 10,000.

SOFTWARE LIMITATIONS

Ballistic Software is just a starting point, but all things equal, you should be within at least 1 MOA of most ranges. It might vary a little, but overall it should be very close. The computer cannot for all the variables and we don’t tell it everything it really needs, which is why we true.

TRUING REVISITED

  1. Calibrate you scope regardless of the brand, testing as much elevation as possible. If you test it across 100% of the useable elevation range you are much better off. Errors with scopes usually have to compound. So the further out you go, the more likely the errors will show up.
  2. Use a Chronograph, and understand they are plus or minus 10% for a decent one, 5% for a really good one and more if you cut corners. Expect to adjust your Muzzle Velocity. Chronographs are subject to a lot of inaccuracies of a variety of reasons. Assume it is off and adjust.
  3. The manufacturer lied to you, that BC you are using is dependent on a velocity you may not be reaching. It was not batch tested, and it might not have any practical real world value at all. So unless you run two chronographs to measure your actual BC, assume this is wrong and will need to be adjusted. Inconsistent weighs, deformed meplats, all exist in the real world.
  4. Shoot on paper and measure your point of aim versus your point of impact. Steel has a lot of fudge room so use a water line if you are checking on steel to confirm your dope. Shoot at far as you can, the more information you have the better.
  5. Confirm your distances as best you can.
  6. Understand we influence the bullet as much as every variable in the books. The reason a lot of the “science” does not affect everyone equally is because of Us the Shooter.
  7. If it works don’t fight it. If you cannot get the entire of flight of the bullet to line up, don’t pull your hair out, make two tracks. A short range one and long range one. If they work, you solved the problem.

These are a starting point you still have to do your part to complete the equation.

For more on this topic, head over the Sniper’s Hide Forum and Visit the Ballistic Computer Section