Topic of the Month: January
2002
Some Comments on Terminal
Ballistics
Terminal ballistics is
concerned with the effect of a projectile on a target or other object. It is deals with
the projectile’s shocking power, penetration, expansion, wound channel, etc. While
the target shooter is little concerned with terminal ballistics, the hunter, military,
police and silhouette shooter are very much interested. For the hunter it is how the
bullet performs that will determine whether the winter meat goes to the freezer or not.
Likewise the military and police need to know the effect of the projectile on the enemy or
the armed criminal.
The basic physical laws of the conservation
of energy show that the bullet weight and velocity determine how much damage is done to
the target. Both momentum and energy are conserved. A bullet leaving the muzzle of a rifle
has kinetic energy (it’s moving and spinning), thermal energy (it’s hot) and
potential energy (it can fall to the ground). From calculations made on total bullet
energy, kinetic energy makes up about 95% of the energy delivered to the target. The other
forms of energy, rotational, potential and thermal energy can be ignored for all practical
purposes.
The Load From a Disk for Windows program,
featured at this web site, calculates both kinetic energy (KE) and the Taylor Index (TI)
anywhere along the bullet’s trajectory. The equations for these energy calculations
are:
KE = m*v2/2 = bw*v2/450436
(1)
Where:
KE = Kinetic Energy, ft-lbs
m = bullet mass, slugs
v = bullet velocity, fps
bw = Bullet weight, grs
TI = bw/7000*v*DI (2)
Where:
TI = Taylor Index
bw = Bullet weight, grs
v = bullet velocity, fps
DI = Bullet diameter, inches
The "Taylor Index" is a measure
of "Stopping Power" of a particular bullet and includes factors concerned with
bullet velocity, bullet weight and bullet diameter. The Taylor Index, is thought by many
ballisticians to be a better gauge of terminal bullet performance than the kinetic energy
alone. The larger the Taylor Index, the greater the stopping power. For instance, the
.30-06 delivers a TI of 19.9 at 100 yards, using the 168 gr bullet at 2693 fps. The .460
Weatherby Magnum delivers a TI of 73.8 at 100 yards, using the 500 gr bullet at 2255 fps.
On the other hand the .223 Remington delivers a TI of only 5.1 at 100 yards, using the 55
gr bullet at 2883 fps.
The rules of interior and exterior
ballistics are pretty well known and the projectile’s performance in the gun and in
the air can be calculated with considerable precision. However, terminal ballistics cannot
be so accurately predicted since every target material creates it’s own terminal
ballistics parameters and the number of potential target materials is virtually infinite.
The basic principles of terminal ballistics
can be reduced to simple terms: "Shocking power" is a function of bullet
velocity and bullet frontal area and penetration is a function of velocity and sectional
density. These hold true only with a non-deforming bullet in a homogenous medium.
Some describe terminal ballistics as
external ballistics in slow motion. The bullet travels through a medium (such as game
tissue) that is denser than air and the bullet loses more energy per unit of travel. The
blunter bullets with poor sectional density tend to lose more energy per unit of travel.
It is not as simple to calculate as it is in air, because game tissue generates enough
resistance to deform most bullets.
The reason for controlled expansion, in
hunting bullets, is to make the energy transfer peak about the same time the bullet is
passing through the vital area of the animal. Not all bullets expand at the same rate, so
a bullet with little energy transfer peaking at the right place may deliver more shock to
vital organs than a bullet with a lot of energy transfer peaking at the wrong place.
To get the bullet in a vital area you need
penetration. As a rule of thumb: velocity times sectional density is equal to relative
bullet penetration. This is valid for bullets of similar composition. In good hunting
bullets, expansion is consistently in the neighborhood of double the diameter and we
assume the bullet will retain its weight. If the bullet goes to pieces, losing weight and
consequently its sectional density, you might as well forget about it. The heavier bullet
has a higher sectional density and will maintain the higher sectional density when it has
expanded to double its diameter. Also the heavier bullets tend to have lower velocity. At
lower velocity the resistance and consequently crushing force on the bullet nose, is
lower, so expansion is delayed. This is why heavy bullets have delayed expansion, and
don’t show the same tendency to blow up on the surface as light bullets even though
they have identical impact energy.
What this boils down to is that the shock
effect and penetration rise directly with the velocity, but the extra 50 or 100 fps out of
a hot, high pressure load can’t give you either the shock or penetration that you
might expect. Undoubtedly, it’s simpler to improve energy transfer by going to a
slightly larger diameter bullet than risking a wrecked gun with a high-pressure load.
The shooter is left with the simple
question; at what point do the terminal ballistic gains to be realized from a heavier,
larger bullet get cancelled out by the exterior losses imposed by the more limited
velocity and ballistic coefficient? You can’t put meat in the freezer if the animal
is too far away to hit in the first place. So …… terminal ballistic principles
don’t tell you what to do to insure perfect success; they tell you more about what
not to do to avoid failure.
Proper bullet selection is easily worth an
extra two or three hundred foot-pounds in hunting, since the right bullet delivers the
energy in the right place to do the most good. For the silhouette shooter, he must choose
a cartridge that delivers the highest kinetic energy to the target and use the heaviest
bullet the cartridge can handle to get the most of his gun.
To get the most from your gun, and from
your loads, you must know your bullet and your target. After that effective results are
much easier to obtain.
Watch our web site for the next topic of
interest "Point Blank Range." Until then, shoot safely and know where you
bullets are going.
Sincerely,
The Ballistician
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