RCBS AmmoMaster Chronograph - Manual (Page 7)

recorded velocities (see Figure 16). Depressing “Enter” again, or
the “Add Data” button, the display will flash between “String” 4 and
“Ready” on “Shot” 1. Now the unit is ready to record shot 1 in string 4.
Figure 16 - Display for No Data in String
SKYSCREEENS
your AmmoMaster ® Skyscreens operate using standard photocell
technology. The photocell is constantly gathering incoming light. When
an instantaneous change in light is detected, the photocell saves the
time at which this occurs. For example, when a projectile is fired over the
Start Skyscreen of the Chronograph, the shadow cast by the projectile
triggers the photocell at a moment in time. The instant the projectile
passes over the Stop Skyscreen, the shadow cast by the projectile
triggers the photocell at another moment in time, thus generating a
start and stop time. Since the Skyscreens are manufactured with a
known distance between them, the projectile velocity can be calculated
using the equation Velocity = Distance ÷ Time.
DIFFUSERS
The diffusers for your AmmoMaster ® Chronograph play a very important
role in recording velocity readings. The diffusers are used to help
filter out any unnecessary incoming light. This aids in the photocell’s
ability to detect a slight change in light as a projectile passes over
the Skyscreen. if the diffusers are not used on a clear, sunny day, the
photocell may be saturated by all the incoming light and may not detect
the shadow cast by the projectile. imagine trying to spot an object flying
through the air as it passes directly in front of the sun; this is similar
to using the chronograph without any diffusers on a clear sunny day.
Again, imagine trying to spot the same object flying through the air as
it passes directly in front of the sun but this time, there is an overcast
condition. The object is much easier to see. This analogy is similar to
using diffusers on your Chronograph.
LIGHTING CONDITIONS
now that you have a basic understanding of what makes your
chronograph function, you know that the lighting conditions are
very important. A Skyscreen’s accuracy and sensitivity to changes in
illumination is based on the ambient lighting conditions. While they
work most of the time, chronographs do have limitations which will
cause errors. Too much light, as well as not enough light, will inhibit the
chronograph operation.
in low light situations, i.e., early morning, late evening, overcast or
cloudy conditions, the chronograph may produce inconsistent velocity
measurements or may not record velocity readings at all. Try removing
the diffusers to allow more light to reach the photocell.
problems can also arise on clear, sunny days when shooting over snow,
water, or light colored sand/gravel, etc. Reflected light off the ground
can illuminate the bottom of the bullet and cancel the shadow cast onto
the skyscreen causing erroneous velocity readings. When this happens,
try covering the ground around the chronograph with a blanket or non-
reflective tarp. This should minimize the reflected ground light and
alleviate the problem. Another technique is to color the bullet with a
black marker, especially if the bullets have a shiny appearance.
in locations with direct overhead sunlight, the Skyscreen may become
saturated by the incoming light, even when the diffusers are installed.
Excessive light can cause a blind condition and the photocell may not
“see” the bullet pass over. This is because the small bullet’s shadow
is not providing sufficient contrast. To reduce the amount of input light,
try covering the diffuser’s top surface with any material that will absorb
or reflect the light (aluminum foil, cloth material, cardboard, etc.) Don’t
use anything heavy enough to disrupt the units balance or damage the
diffuser or Side Supports. if all else fails, it might be best to come back
and shoot another day or wait until later in the day when the light has
subsided.
INDOOR SHOOTING
To use indoors, you must provide the chronograph with an artificial light
source. Install a standard incandescent light bulb centered 24 inches
above each diffuser to provide a uniform illumination. We recommend
using a 60 – 100 W bulb and an aluminum reflector dish for each
light fixture, if possible. These products are readily available at most
hardware stores.
Other lighting configurations may provide acceptable results (different
installation height, no reflector dish, etc.), but be slightly less accurate.
fluorescent light sources will not work reliably because they do not
provide a continuous light source.
SHOTSHELL VELOCITY
it is highly recommended that you determine your shot’s pattern
size before starting. Only use your Diffusers and Side Supports if your
gun’s shot pattern is small enough to pass through the chronograph’s
shooting area without causing damage. fAiluRe To do So MAy CAuSe
SeRiouS dAMAge To The SkySCReen oR oTheR CoMponenTS And
poTenTiAlly CAuSe SeRiouS inJuRy To The ShooTeR.
To prevent damage to your Chronograph, we recommend removing
the diffusers and Side Supports when measuring shotshell velocity.
Position weapon 10 feet away from chronograph (see Figure 9) and
continue using as you would for centerfire rifle ammo. Certain ambient
and environmental conditions can result in erratic measurements when
the diffusers are not used. See “lighting Conditions” section for more
information.
To measure your shot pattern, place a paper target approximately 15
feet from the muzzle of your firing position. Shoot into the target to
measure the diameter of the shot pattern. Then, determine a desired
aiming point to ensure you are shooting approximately 4-6 inches
above the chronograph Skyscreens. once this has been done place
your chronograph so the back of the unit is against the target. Verify
the shot pattern is the correct height above the skyscreens. Move the
target a few feet further away (ie., down range) from the chronograph
and begin recording velocities.
ARROW VELOCITY
for better measurement accuracy, make sure you are far enough away from
the chronograph so that the arrow separates completely from the bow string
before the tip reaches the “Start” skyscreen. Blunt arrow tips work best.
STATISTICAL DATA
Statistical data only applies to the recorded data within a selected shot
string, not multiple strings. Therefore, the largest sample size for any
statistical data measurement will be a sample size of ten.
High Velocity
displays highest recorded velocity within selected shot string.
Low Velocity
displays lowest recorded velocity within selected shot string.
RCBS AmmoMaster ® Chronograph Instructions 7