ORANGE
DIFFUSERS
CAN'T
ALWAYS
HELP
The orange diffusers shade the lenses from noonday sun and make the hot-
spot light from the sun into a uniformly bright orange background against
which the eye can see the bullet.
If
you are shooting under cloudy skies,
with heavy overcast, or in the shade, then no direct light strikes the diffuser.
What little light is available
is
already diffused, and the diffuser can't make it
any brighter.
If
the orange diffusers don't cast a shadow, and you have
problems,
try
leaving them
off
. You can still use the black side-rails
as
aiming guides.
Expect trouble
if
you
try
to shoot under the shade
of
a tree. The sun can cast
a spotted shadow on the diffuser; the bullet may go through a bright spot and
be detected or the bullet may go through a shaded spot and be
missed. With
no diffuser, the eye may see the bullet silhouetted against a spot
of
sky, or the
bullet might be silhouetted only against the black bottom
of
a limb.
SUBSONIC
VEWCITIES
With velocities below the speed
of
sound, the muzzle blast wave reaches the
screen before the bullet. This muzzle blast wave
is
like a lens traveling
through the
air at the speed
of
sound, and the resulting light diffraction can
trigger the skyscreens. (The speed
of
sound is approximately 1060 plus the
air temperature in degrees Fahrenheit, or 1130 fps at room temperature
.)
Premature triggering
of
only the start screen will cause both velocities to be
abnormally low and the difference to be high. Premature triggering
of
both
start and middle screens will cause the primary velocity to read abnormally
low and the proof velocity to read the speed
of
sound. Premature triggering
of
all
three
screens (a rare case) will cause both primary and proof velocities
to read the speed
of
sound.
If
you see the effects
of
blast, the only sure solution
is
to install a blast baffle
midway between muzzle and flIst screen. This baffle should be a piece
of
plywood with a small hole or vertical slot to shoot through. We use a 2x4'
piece
of
plywood standing on end and supported
by
a single hinged leg. Cut
a vertical inch-wide shooting slot from bench level to the top
of
the shield.
Occasionally blast problems can be cured by moving the skyscreens farther
from the muzzle
or
by slightly changing the shooting direction.
Page
18
BOWS
The M35P with Sky screen III detectors works well for archery. Make sure
that the screens are far enough away that the arrow leaves the string before
the point gets to the first screen. Blunt points work best, followed by round
points and target points. Needle-like field points and broad-heads can give
erratic results. The skyscreens need to see an abrupt change in light.
Because arrows are slower than bullets, you must help the skyscreens by
using a flatter nose.
SHOTGUNS
The system can be used for shotshells. It reliably measures the velocity
of
the front pellets in the shot string. These velocities are typically 2% to 5%
higher than factory measured velocities. The factory systems (also made by
Oehler) use inductance sensing coils to measure the velocity
of
the large
clump
of
shot at the back end
of
the shot column. A primary screen spacing
of
four feet is best for shotgun work. The first screen should be placed three
or four feet from the muzzle.
You will get the most consistent and accurate
results
if
you use the most open choke available.
You
must
wear
safety goggles while shooting!
INDOO
R
SHOOTING
To
use skyscreens indoors, you must provide a substitute for the daylight.
Use a reflector type incandescent lamp shining down on the top
of
the
diffuser
of
each skyscreen. You must use incandescent lamps; fluorescent
lamps will not work! Mount thc lamps just high enough to uniformly
illuminate the diffuser. Common utility reflectors with 60 to
100 watt bulbs
work fine. The 90 or
150 watt PAR outdoor reflector lamps with their heavy
glass cnvelopes will better resist muzzle blast.
Using the Skyscreen III units
indoors with lamps will work most
of
the time. Adding lamps to our
skyscreen type system will not give you all the advantages
of
our laboratory
type systems. Our laboratory systems cost more, and they perform better.
Page 19
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