Let Fly Ballistics Calculator Deluxe

Welcome. Let Fly is a ballistics calculator for modeling the flight of a rifle bullet. Let Fly is useful for adjusting aim when making long shots in all weathers. Let Fly is simple to use in the field. Let Fly Deluxe adds internet weather, iPhone sensor inputs, graphic crosswind calculation to the original rock solid Let Fly simulator. We hope you enjoy it.


To use Let Fly select a rifle from the list, provide the target and weather details of the shot. Let Fly automatically displays the drop and drift for the bullet at the target. Drop and drift are expressed in inches and in scope turret clicks.

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The Shot screen, and other ballistic input screens display the results at the bottom.

When inputs change, the results display in gray as shown above. Let Fly works in the background to recaclulate the ballistic simulation whenever the inputs change.

When the results are current, they are displayed in black as shown in the example above.

The Report button will produce a screen with the full details of the shot, including weather station and sensor info, a drop chart and an impact chart.

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Ballistics Inputs

There are more than a dozen different inputs to the Let Fly ballistic simulation. They are summarized on The Shot screen. The inputs can be adjusted on the four input screens. The details of each of these inputs is discussed in later sections.

The rifles in the Rifle List include a profile containing the contents of the Bullet screen and the Sights screen. A Rifle Profile is the collection of details from the rifle sight geometry, bullet drag, weight and muzzle velocity.

Target and Weather conditions are input thru the following screens.

Target inputs are supported with a screen for computing crosswind, and additional screens for using camera input and estimating range. The Target Camera captures the shot incline and heading.

On the iPad the ballistic inputs are shown on just two screens.

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Slider Controls

Let Fly uses slider controls for many of the inputs. Often the value range represented by the slider requires the slider have coarse sensitivity.

Adjacent to the sliders are Plus and Minus buttons. These two buttons provide fine adjustment for the slider.

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Bullet Details

To see the Bullet Details screen tap the Bullet button on The Shot screen.

There is a wealth of bullet ballistics information available from ammunition makers and reloading suppliers. Ammuntion packaging often has the bullet weight and muzzle velocity.

More detailed information including the ballistics coefficients are available from reloading manuals published by several manufacturers. The internet also has a wealth of information on rifle shooting.

Bullet Details Controls

The flight of a bullet is modeled in Let Fly by its three characteristics: muzzle velocity, weight and ballistic coefficient.

Muzzle Velocity Slider

This slider allows the Muzzle Velocity for the bullet to be adjusted up or down.

Weight Slider

The Bullet Weight is in grains. The grain is a traditional unit of measure used for bullets. There are seven thousand grains to the pound.

Ballistic Coefficient Slider

The Ballistic Coefficient is used in conjunction with one of the G tables to find the drag on the bullet during the flight simulation. The generic rifles in the list provided by Let Fly all use the G1 table which has been the standard since the late nineteenth century.

Ballistic Table Selector

The G5-G8 tables are specialized models of the newest bullet designs.

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Sight Details

To see the Sight Details screen tap the Sights button on The Shot screen.

Sight Controls

The sights are defined by the zero in range and the height of the sight above the bore centerline. The denomination of the scope adjustment click is also needed to provide clicks to the results.

Zero in Range Slider

The Zero in Range is in yards or meters. The slider has a 25 yard/meter granularity. Selecting meters sets Let Fly to metric throughout.

Sight Above Bore Slider

The Sight Above Bore distance is in inches, and is measured from the center of the scope or sights to the center of the bore.

Scope Click Type Selectors

Many rifles have telescopic sights with adjustment turrets. The turrets have detents which make turns denominated in clicks. Adjusting the turrets makes an angular adjustment to the sight, either vertical or horizontal, depending on the turret. This angle is expressed either in minutes of arc or milliradians.

NONE indicates a rifle without turret click adjustable telescopic sights.

MOA indicates minute of arc. There are 21,600 minutes in a circle.

MIL indicates milliradian. There are about 6283 milliradians in a circle. A MIL is about three times as large an angle as a MOA.

One, 1/2, ... 1/10 is the fraction of a MOA or MIL. A very popular click size is one quarter MOA.

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To see the Weather screen tap the Weather button on The Shot screen.

Weather Controls

Weather is described in Let Fly with four inputs: elevation, barometric pressure, humidity and temperature. The default weather settings are the Standard adopted when the original G1 table was developed, a rather cool damp sea level.

Elevation and Temperature are the two most important of the weather inputs.

Apply Button

The Apply buttion applies the latest weather observations from the nearest reliable station, usually an airport. After applying the latest web request, a new web request is started.

Using web weather requires first a GPS lock on your location, and second a message roundtrip to the web via the iPhone. This may take a few seconds. The station name will remain gray until a report is received. You may need to tap the Apply button again after a second or so.

Elevation Slider

Elevation affects the air pressure and density. Bullets fly more efficiently at higher altitudes. When the elevation is adjusted, the barometric pressure slider is automatically adjusted to standard.

Barometric Pressure Slider

Lower air pressure improves bullet flight. Set the Let Fly elevation before setting the local barometric pressure slider. If the local pressure is not known, use the standard pressure provided by the elevation slider.

Humidity Slider

Humidity improves bullet fight, but is less important at cooler temperatures. At high temperatures high humidity can be significant. If the local humidity is not known, use 30% for dry, 50% for fair, and 90% for wet weather.

Temperature Slider

Temperature is important for modeling. Higher temperatures improve bullet flight. The input range is from -40F to 120F. If the local temperature is not known, try to estimate within 10 degrees or so.

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To see the Target screen tap the Target button on The Shot screen.

Target Controls

The Target inputs are distance, incline and crosswind.

The Camera can be used to capture the incline and heading of the target. The heading is used by the crosswind calculator.

Incline Slider

The Incline ranges from -50 degrees downhill to +50 degrees uphill. The camera will set the incline at the snap.

Range Slider

The Range is in yards or meters. The slider has a 25 yard/meter granularity. For finer grain adjustment of the range use the plus and minus buttons. The yards/meters setting is controlled on the Sights screen.


Crosswind is the component of the wind which blows at 90 degrees to the flight of the bullet. Use the Wind button to summon the crosswind calculator screen for setting the crosswind. Drift is derived from the crosswind. When the crosswind is zero, the drift will be zero.

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Target Camera

The Target Camera uses the iPhone camera to capture the incline of the target. If the iPhone also has a compass, the heading is also captured.

Here are the steps for using the Target Camera

All of the camera inputs can be reset and adjusted later in the Target and Wind screens.

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Crosswind Calculator

The Wind screen computes the crosswind to the rifle. The situation is depicted with a wind rose with a wind sock at its center.

Wind Slider

The Wind slider orients the windsock in the center of the wind rose, and adjusts the wind direction. The clock ring of the wind rose shows the relationship of the wind to the rifle. The Apply button will set the Wind slider per the nearest weather report.

Speed Slider

The Speed slider scales the windsock in the center of the wind rose, and adjusts the wind speed. The Apply button will set the Speed slider per the nearest weather report.

Aim Slider

The Aim slider sets the compass heading of the target from the rifle. The outer ring of the wind rose turns with this slider. The Target Camera also sets the aim.

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New Rifle

Once the settings for Bullet Details and Sight Details are as desired they may be saved as a new rifle profile. It is good practice to save a new profile to match your rifle, its sights and cartridge.

The Save as button on The Shot screen creates the new rifle and displays a text box to enter the name for the new rifle. The default value is the same name as its predecessor with "new" appended. Tap in the text box and the keyboard appears. Edit the name using the keyboard then tap Done to finish. Tap The Shot to continue.

If you get the name wrong, create another new rifle with the correct name. Then delete the incorrectly spelled entries using the process described in Edit Rifle List.

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Edit Rifle List

As new rifles are defined, they appear at the top of the Rifle List. To remove unwanted rifle from the list, tap the Edit button.

Tap the No Entry icon to unlock the rifle to be deleted.

The lock icon will turn to the vertical orientation, and a red Delete button appears.

Tap the Delete button to confirm the rifle delete.

Tap the Done button to finish editing the list.

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Help Screen

The About button at the top of the Rifle List summons the Help screen, which is this web page.

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