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Bullet
Data, Field Conditions, Target Data, And Results
Each of the three primary forms has fields to enter data and push
buttons to move onto the next logical form or to return to the
main form and menu. The first logical form is the “Bullet
Data" form, which has fields to specify bullet weight,
sight in distance, distance between line of sight and line of
bore, and the ballistic coefficient of the bullet.
A check box is used to specify that the sight in distance
should be meters rather than yards. Push buttons are used
to go to the next logical form or to the main form.
The
second logical form is the “Field
Conditions” form, which has input fields to specify muzzle
velocity and current atmospheric conditions at the target location.
These include altitude, temperature, pressure (not corrected to
sea level), and relative humidity. A check box is used to
indicate that system should calculate pressure based on altitude.
The
third logical form is the “Target
Data”
form, which has input fields to specify target distance, wind
speed, wind direction, target speed, and incline angle to target.
A check box is used to specify that the target distance should
be meters rather than yards. A push button is used to start the
ballistic calculations. Another push button is used to return
to the main form and menu.
Bullet Data
Field Data
Target Data
Ballistic Coefficients
Optional
Input Forms
Sight In Data, Ballistic Coefficients, Sight Adjustment
Options
Three optional forms are provided to enable additional specifications.
The "Sight
In Data" form is used to specify the conditions at the
time that the rifle was sighted in which may be significantly
different than the conditions at the target location.
There are input fields for muzzle velocity, and atmospheric
conditions at the time of sight in.
A check box (automatically set by default) is used to indicate
that the sight-in conditions are the same as those specified under
“Field Conditions” above.
The multiple "Ballistic
Coefficients” form is used to specify bullets that have ballistic
coefficients that change with velocity. Examples of these are
the high efficiency Sierra MatchKing target bullets. The
30 caliber 168 grain MatchKing is illustrated in the following
display.
The interpretation of this data is as follows:
For velocities exceeding 2600 fps the ballistic coefficient
is 0.462, for velocities between 2600 and 2100 fps the ballistic
coefficient is 0.447, for velocities between 2100 and 1600 fps
the ballistic coefficient is 0.424, and for velocities less than
1600 fps the ballistic coefficient is 0.405.
There is an excellent discussion in the Sierra Reloading
Manual of how the need for multiple ballistic coefficients was
discovered.
The fact that the ballistic coefficient changes as velocity changes
is taken into account when Exbal performs the ballistic motion
equations.
The "Sight
Adjustment Options" form is used to specify the units
of sight correction for elevation, windage, and target lead (for
moving targets). In the display of results, both inches and “clicks”
are displayed. The default units of sight adjustment (clicks)
are one minute of angle (approximately one inch per hundred yards).
Many target scopes have dial readings for windage and elevation
in minutes of angle. Other scopes have click stops for 1/8,
¼, ½, or one minute of angle. Still other scopes have internal
gradient lines or dots to indicate minutes of angle and/or mil-radians
(mil dot).
Sight Adjustment Specifications
Sight-in Conditions
The Pocket PC version has two data
look up functions. The first one provides look up of ballistic
coefficients provided by the bullet manufacturers. The second
one provides look up of velocity data for factory loaded ammunition.
Look up Ballistic Coefficients Look
up Velocity Data
Exbal provides two output windows.
The "calculate" button shows the ballistic data for
the specified target. It contains all of the information about
the bullet at the target point. The "drop table"
button provides the sight adjustment data needed at a series of
target distances.
Calculation Results
Drop Table
Exbal Utility
Functions
Three "utilitiy" functions
are provided to enable the shooter in the field to make adjustments.
First there is the Match Sight-in Point
function which calculates the true zero point based on the bullet
impact at a specified distance. Second there is the Trajectory
Correction Function which uses a known sight-in distance
(zero point) and calculates the velocity that matches impact point
at a known distance. And finally there is the Range
Estimation Calculator which does the math associated with
using a ranging reticle to estimate target distance.
Match Sight-in Point
Trajectory Correction
Range Estimator
Ballistic
Reticle Analysis For The Pocket PC Version of Exbal:
Scopes with ballistic
reticles are increasingly being used by long range shooters and
they have been used by military "snipers" for many years.
They offer an alternative to making external scope adjustments
to compensate for bullet drop at longer ranges.
The concept of the ballistic reticle is introduced on http://www.perry-systems.com/reticle_analysis.htm
web page. A ballistic reticle consists of a main cross-hair
and a series of aiming reference points (bars, circles, or dots)
which are spaced along the vertical cross-hair.
Information about specific reticles can be obtained from
Swarovski/Kahles, Burris, Leupold, Nightforce, and other vendors
who offer them with their scopes.
To illustrate this concept, the set up window and point blank
range analysis results are shown for a 308 Win, 175gr MatchKing
bullet at 2600 fps, sighted in at 100 yards, shooting at a target
600 yards away. The
reticle being illustrated is a Nightforce NP-R2 reticle, which
has 10 reference bars spaced at 2 MOA intervals.
The Point Blank Range results for this particular case indicate
that Bar #8 should be used. (the zero point for Bar #8 is
592 yards so the bullet will strike about 2 inches low)
The beauty of this approach is that no external scope adjustments
were needed.
Ballistic Reticle Analysis
Point Blank Range Analysis
Now
the Palm version of Exbal will calculate how the use of a ballistic
reticle will affect the bullet impact point compared to the aiming
reference point used.
There are new windows to support this analysis. The first
window enables the user to select retcle data from a list of reticles.
The second window is used to manually specify the configuration
of the ballistic reticle being used.
This is where the measurement system (MOA, IPHY, or MILS)
and the angular difference between the main cross-hair and each
of the aiming reference points (bars, circles, or dots).
For simplicity the reference points are labeled Bar #1,
Bar #2, etc.
There are three different measurement systems used to specify
the angular difference between the horizontal cross-hair and the
aiming reference points (bars, circles, dots).
First there is the traditional "minute of angle"
or MOA which is equivalent
to 1.048 inches per 100 yards.
The second system is "inches per 100 yards"
or IPHY.
And the third system is "mil-radians"
or MILS (1 meter per
1000 meters) which is equivalent to 1 yard per 1000 yards or 3.6
inches per 100 yards.
Ballistic Reticle Selection List
Edit Reticle Specifications
Two optimization functions have been
added. The first function determines what sight-in distance
should be used so that a particular bar will strike the target
center at the specified target distance. The second function
determines what operating power the scope must be set at to achieve
making the zero point of the specified bar equal to the target
distance. The second function is only applicable to scopes
that have the reticle in the second focal plane. If the spacing
of the bars or dots remains constant while the image size changes
with power, the reticle is in the second plane.
Calculate Optimum Sight-in Distance
Calculate Optimum Power Setting
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