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Twist Rate Calculator
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Quantity in Basket:
None
Catalog No.: DC-TWIST
Price: $12.00
Shipping Weight: 0.10 pounds
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The DC-TWIST program helps you calculate the
proper spin rate (rifling twist) for any bullet, as well
as the RPM of the bullet at any velocity. Unlike the
conventional Greenhill formula, Corbin's twist rate
calculator factors in differences in jacket, core,
and tip density in English or Metric units, as well
as velocity and center of gravity, and allows change
of media through which the bullet travels.
You can calculate stable twist rates for firing at
different air densities, through gasses, under water,
etc.
The program instantly changes all figures from
metric to English or vice versa, at the click of a
button. You can convert meters per second to feet
per second, grams per cubic centimeter to pounds per
cubic inch. A pop-up calculator window converts various
density and specific gravity values for you, and
provides a list of media, core, jacket and tip
materials with their density. Clicking one of these
values, and then clicking "calculate" displays the
value in gm/cc, lb/cubic inch, SG, and other equivalents.
Clicking a button for core, jacket, tip, or media
transfers the density you have selected back to the
main program window and inserts it into the selected
component entry box for you.
You can save all the figures in a file by clicking a
button, and print either the saved info as a report,
or print the window just as it appears without saving
any data. You can also enter data and then save it as
the new defaults, so that you can customize the
program to remember and recall your own figures
instead of its original defaults.
DC-TWIST can be run from the CD-ROM (without saving
reports or defaults) or installed on your hard drive,
using the self-starting menu on the CD-ROM.
Just put the CD-ROM in your drive, and the menu will
appear (if your system has been set to allow auto-start:
otherwise, click CDM for CD Menu). No technical expertise
is required to calculate the proper twist rate for
any bullet! At the price, any bullet maker should have
this useful tool in their arsenal.
The program adjusts the spin rate required for very
low and very high velocities, since the rate of spin
generates different centrifugal forces on the bullet
as the velocity changes. Once the bullet is free of the
barrel, the velocity has little effect on required spin,
but since velocity determines the bullet rotational
speed imparted by the rifling twist, a higher twist
rate would be required to stabilize a bullet fired at
very low speed compared to firing the same bullet at
a high speed. This is a neglected factor in conventional
Greenhill calculations. Obviously, at zero forward
velocity there is no rotation imparted, and at very
low forward velocity, the same numbers of turns per
unit of distance generates a very small number of
revolutions per minute, but the usual method of
calculating stable twist has no input factor for
varying velocity.
Likewise, a bullet with the center of gravity shifted
far to the rear will require much higher spin rate to
maintain nose-forward attitude compared to a bullet
with the CG shifted further forward. Most bullets
have their center of gravity somewhat aft of the linear
midpoint. If dropped from a height with the nose
pointed down, they would land base first.
A nose-heavy bullet, on the other hand, would land
nose first. In flight it would require less spin to
maintain nose-forward attitude.
With the CG moved too far forward, the bullet would
require little or no spin to strike nose first, but
would follow the launch attitude over its trajectory
path (that is, the nose would continue to point in the
direction and angle at which it left the barrel,
so that as the trajectory arc became larger with
greater distances, the bullet would tend to fly at
an angle to the direction its nose pointed, and
would begin to tumble).
DC-TWIST considers the location of the bullet center of
gravity, so that with all other parameters equal,
a bullet with the weight shifted forward, such as
a hollow base wadcutter, an airgun pellet, or a
hollow base shotgun slug, would require less spin
than a bullet having a long pointed nose and a
flat solid base. Likewise, bullets composed of
different density materials, such as tungsten powder
base sections and plastic nose fillers, would
require different spin rates than the same weight and
size of bullet with the same average density in a
monolythic core design.
The program allows for
location of the CG relative to the center of form,
as a percentage from the base to the tip. Entering a
value more than zero and less than 100 lets the engineer
shift the balance of the bullet and determine the
effect on stable spin rate.
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