bullets summary

[Jesus Dapena]

Dear Biomch-L readers:

I got many responses to my query about why the back of a bullet is
flat instead of tapered. Thank you to all who responded!

Here is my original message:

> Dear Biomch-L netters:
>

> This is a question about mechanics, rather than BIOmechanics, but
> since the answer may affect principles that also apply to
> biomechanics, I feel that it is appropriate for this forum. A couple
> of days ago, in a biomechanics class that I was teaching we were
> covering the topic of "Mechanics of Fluids", and one of the students
> asked me the following question:

>

> "Why is the rear end of a bullet flat instead of tapered? We
should
> expect a hypothetical bullet with a tapered rear end to receive a smaller
> drag force, and therefore to maintain its velocity better and to reach a
> longer range."
>

> I was unable to give a good answer. I can see how the back of the
> shell CASING may somehow need to be flat, to ensure a good impact that
will
> guarantee the ignition of the powder, but this does not explain why the
back
> of the bullet itself would have to be flat.
>

>

> Does anyone out there know the answer to this one? Please send
your
> messages to me, and I'll post the collated answers.
>

> ---
> Jesus Dapena
> Department of Kinesiology
> Indiana University
> Bloomington, IN 47405, USA
> 1-812-855-8407 (office phone)
> dapena@valeri.hper.indiana.edu (email)

I am giving first my own summary of the answers, which covers most
of the comments and theories that I received, and then I will post the
actual messages that I received from individual people, where you will find
other theories and much detailed information.

We could divide the reasons for non-tapered bullets into two
categories: (a) reasons related to the flight of the bullet; (b) reasons
related to the impulse given to the bullet inside the gun barrel.

(a) reasons related to the flight of the bullet:

(1) The bullet moves so fast (supersonic in many cases) that there
is aerodynamic separation almost immediately after the tip. Therefore, it
is irrelevant whether there is tapering of the back or not. Tapering of the
back would help only in slow-moving bullets (slow either through slow
initial muzzle speed or through slowing down after a long distance of travel
through the air).


(b) reasons related to the impulse given to the bullet inside the gun
barrel:

(1) The base of the bullet is more or less flat so that the
explosion expands the relatively soft bullet to make it "hug" the inside of
the barrel. This prevents leakage of burning gases, and also helps the
bullet to grip the helical grooves of the barrel (which will provide spin,
and therefore stability during flight).

(2) Ease of manufacture.

(3) A boat-tailed (i.e., non-flat) bullet would make the forces be
directed in a diagonal direction (forward and toward the center of the
bullet) instead of directly forward. Therefore the FORWARD component of
force would be smaller. (NOTE: Although this was a very popular response,
I don't think it is correct. It is true that a boat-tailed bullet would be
subjected to forces that would be pointing less directly forward, but the
total area of contact between the gases and the bullet would also be larger.
A quick trigonometric calculation shows that the forward component of force
would be the same, regardless of the shape of the back surface of the
bullet, as long as the caliber stays the same --and as long as no gases are
allowed to escape between the bullet and the barrel. The forward force
would be equal to P x A, where P is the pressure of the exploding gases, and
A is the cross-sectional area of the barrel.)


An interesting point: I found through the responses that some
bullets ARE tapered in the back! This may be useful in slow-speed
(definitely non-supersonic) guns, or in the case of a nearly "spent" bullet
after it has lost much of its initial speed through air resistance.

Here are the individual responses:

---------------------------------------------------------------------------
from "Mark D. Grabiner"

Jesus,

It seems that the rather large force of the within-cartridge explosion that

transforms the bullet into a high-velocity projectile in a very small

elapsed time would tend to flatten its lead composition anyway.


Mark
---------------------------------------------------------------------------
from MWPOWELL@ecl.psu.edu

Jesus,

Regarding your question about the bullet, I would think that it is so the
pressure from the "explosion" has a good bearing surface. If the back end
was tapered, I don't think you would get as much of a push, thereby
causing the bullet to actually travel slower.

Mark

---------------------------------------------------------------------------
from jerry@mlink.COM (Jerry Nicholson)
Hello Jesus,

There are such bullets. They are usually referred to as
"boat tail" bullets and do indeed retain higher downrange velocity
and associated impact force.

The base of many bullets is wide and even slightly concave in order
to help seal in the burning gases at ignition. The hollowed out
base will expand to the inside diameter of the barrel. I'm not
quite sure how boat tails obtain as good a seal unless a plastic
base cup is employeed in a manner similar to that used in shotgun
shells. Boat tails are used almost exclusively as rifle bullets

where retained downrange velocity is critical, and even then are
usually only used in very high velocity calibers such as .270,
223 and 30.06.


Hope this helps.

jerry
---------------------------------------------------------------------------
from craig mcdonald

Jesus,

Looks like you need to break out the wind tunnel. Seems to me the
additional weight and corresponding decrease in space for powder would
be a trade off with the aerodynamical advantage. Very small
cross-sectional area, but very high velocity; what do the equations
give for the drag force?

Cone shaped back of bullet might direct forces outward, decrimping the
casing, resulting in an ineffective transfer of the force to the
bullet from the exploding powder.


The bullet is also rotating due to the rifling in the barrel, does
that affect the laminar flow? More questions than answers.

If it really is better, patent it quick, and sell it to the military
and retire early.

Interesting,

Craig McD
---------------------------------------------------------------------------
from Duane Knudson

I do not have any expertise in this area (A Texan who doesn't own a gun),
but

I suspect that the flat rear of a bullet allow all the pressure from the

propellant to be effectively applied at right angles. The the bullet is,

therefore, driven in the desired direction with the maximum force. A
tapered

bullet may not be accelerated by an even pressure buildup as the propellent
is

discharged. Even if the pressure build up was even (sides balanced,

preventing a tendency to tumble), I suspect it would be less effective
since

all of the propellant force is not in the desired direction.

Duane Knudson, Ph.D.
Department of HHPR
Baylor University
---------------------------------------------------------------------------
from JPILGER@Ness.ScottLAN.Edu

Jesus,

Here's a guess about why bullets are flat at their posterior end.


Maybe it's because the casing can't grip tightly to a tapered end.

One wouldn't want the bullets falling apart before use.


This, of course, doesn't rule our the casing attaching to the cylindrical
region and

the taper extending further back into the shell. It would mean that the

bullets would have to be bigger (greater mass) and that the shell

would need to be larger (to accommodate equivalent amount of powder).

The loss of space for powder might be offset by improved projection

which might be offset by greater mass.

Another thought, would the explosive force be just as great on a smaller

posterior end (due to the taper) as on the larger, untapered

posterior end? Would some of the force "escape" along the side

between the bullet and the shell during the expolosion?

Remember, these are just guesses from a non-biomechanic. I will leave

it a ballistics expert to give a definitive answer.

---------------------------------------------------------------------------
from tellner@cs.pdx.edu

Ask this question on rec.guns. A lot of knowledgeable people
will be more than willing to help you.

Todd
---------------------------------------------------------------------------
from Philip Schot

Perhaps the flat rear surface presents a greater area for the discharge,
thereby acquiring greater speed? With an aerodynamic design, might
the gasses flow around rather than propelling the bullet?

It would be interesting to examine which design would prove most effective
for range/accuracy issues.

phil



:::::::::::::::::::::::::::::::::::::::::::
:: Philip Schot, Ph.D. ::
:: Biomechanics Lab Director ::
:: Department of Human Kinetics ::
:: University of Wisconsin - Milwaukee ::
:: P.O. Box 413 ::
:: Milwaukee, WI 53201 ::
:::::::::::::::::::::::::::::::::::::::::::
:: Phone: (414) 229-6080 ::
:: Fax: (414) 229-5100 ::
:: Email: pschot@alpha2.csd.uwm.edu ::
---------------------------------------------------------------------------
from deleva@risccics.ing.uniroma1.it

Hi Jesus,

I have some guesses that might answer your question. I think that
the bullet is so fast that the aerodinamical separation happens
so violently, and creates such a turbolence, that the bullet wouldn't

have any benefits form a tapered back.
On the other hand, the bullet needs to have a long enough
cilindrical
surface to be correctly guided through the gun, with its long axis
parallel to its direction. Thus, a tapered back would needs increase its
length
and possibly this would require a more complex mechanism in the gun for
inserting the bullet into the barrel.
That's not all: I also hypothesize that the largest percentage
of the impulse that the bullet receives is occurring when the bullet
is still in its "case", where the powder is compressed and explodes.
That is because the barrel cannot have exactly the same diameter as
the bullet, while the "case" does. This means that the gasses expanding
as a result of the explosion will possibly reduce significantly their
push immediately after the bullet goes completely out of the case.
In fact, the gasses could expand with a forward velocity faster than
the bullet when they find space to seep between the bullet and the barrel.
(I didn't think it was so difficult to explain this simple idea;
and it's not finished yet!). With a tapered back, the instant when
the gas might seep between bullet and barrrel would be anticipated.
(at this point, I am sure you understood, and I don't have to complete
the description of this hpothesis).

Last idea. This is really hypothetical. I don't have any
idea about the quantity of powder needed inside a bullet. Anyway,
let's say that the powder occupies 1/4 of the length of the bullet.
This would mean that there is not space for a long tapered tail
(assuming that the tail has to be long to have an effect, IF it

can have any effect, and I sincerely doubt it).

Ciao,


Paolo de Leva
Istituto Superiore di Educazione Fisica
Biomechanics Lab
Via di Villa Pepoli, 4
00153 ROME
ITALY

Tel: 39-6-575.40.81
FAX: 39-6-575.40.81 (or 39-6-361.30.65)

e-mail address: DELEVA@RISCcics.ing.uniRoma1.IT
---------------------------------------------------------------------------
from "Alan Walmsley"

Dear Jesus,

It seems to me there could be a number of reasons for bullets being

non-aerodynamic. The most obvious is that it would not

significantly improve the performance of the bullet for its intended

task. However, other important factors could be:

1. The need to provide some stabiltiy and lubrication in the barrel to

prevent jamming;
2. Many bullets are supersonic in the initial stages of flight, and so

low speed aerodynamics are irrelevant;
3. Many bullets are intended to become unstable as their speed

reduces at the end of their range.
Alan Walmsley
School of Physical Education
Division of Sciences
University of Otago
Dunedin, New Zealand.
Ph (03) 4799122, Fax (03) 4798309
---------------------------------------------------------------------------
from Randy Jensen

Jesus,

Regarding your bullet question: The quickest answer that came to me
was that a flat back end would not allow dispersion of the force from
the explosion, whereas the tapered one might. I may be way off base,
as I haven't really thought it out.

Randy Jensen JENSEN@COE.UNT.EDU
Dept. KHPR
University of North Texas
Denton TX
1-817-565-3424
---------------------------------------------------------------------------
from Luis Aragon

How about this: the end of the bullet needs to be flat so that the
explosion of the powder has a flat surface to "push against" and
propell the bullet out of the gun.

Luis Fernando Aragon-Vargas, PhD Phone & Fax +506-227-9392
School of Physical Education e-mail: laragon@cariari.ucr.ac.cr
Universidad de Costa Rica
---------------------------------------------------------------------------
from Favre@aol.com

Mr. Dapena,
You asked the right question at the right time. Yes the taper has a
great effect on the bullet. Just this past weekend deer season opened in
the
great state of Mississippi. Now if you examine the bullet from a riffle,
you
will notice that it is tapered. In shotguns, slugs are also tapered. Even
though the casing is round, some slugs are in a plastic covering. That
covering then leaves the slug as the slug leaves the barrel. Just ask any
redneck, and we can tell you that rifle and shotgun bullets and slugs are
tapered. With had guns it tends to be a different story. The range and
accuracy of a handgun do not have to be of great precision, therefor there
is
no need for tapering.
-Christopher
Favre
---------------------------------------------------------------------------
from HowieDBPG@aol.com

While I am not sure of this, my best guess would be to have a uniformed
shaped area against which the explosive force would be applied. A flat
surface would certainly solve this problem.

Howard
---------------------------------------------------------------------------
from y-hkwon@kssisun.kssi.re.kr (Young-Hoo Kwon)

Dear Jesus:

I am not quite sure if my idea is right, but let me try.

I could think about two possible answers:

1. For effective thrust of bullet. The tapered shape is good for reducing
drag, but not quite efficient for propulsion. When the direction of push is
perpendicular to the back face of bullet, the thrust will be the most
effective.
If the bullet has tapered back face, the push due to ignition is
perpendicular
to the surface around the backface which eventually is not parallel to the
desired direction of the bullet.

2. Amount of gun power can be stored. If the size of the shell is constant,
you will be able to store less powder with tapered back.

I hope to see better answers.

Peace!

Young-Hoo Kwon, Ph.D.
Senior Researcher
Korea Sport Science Institute
223-19 Gongneung-2-Dong
Seoul, 139-242 KOREA
Phone: +82-2-970-9555
Fax: +82-2-970-9502
Internet: y-hkwon@kssisun.kssi.re.kr

---------------------------------------------------------------------------
from Tec.Serv@Latrobe.edu.au (Technical Services Unit, Carlton)

They earn't all flat. Some are round eg ball shot some have a cup to give
a small lip which expands to fit the barrel eg air guns.

John Yelland
---------------------------------------------------------------------------
from "O.O.E.C"

My guess would be that the end of the case
is flat because you hit it to ignite
the explosives and the end of the bullet
is flat because the gases need to push against
something to get the bullet moving. If it

wasn't the forces would go to the edge of the

bullet and the gas seal would heave to be
much better.
---------------------------------------------------------------------------
from LEGNANI@icil64.cilea.it

I am sorry I do not have t
he aswer...

I am a peacifist.... I prefer peace than war.

bye

giovanni legnani
---------------------------------------------------------------------------
from lk1boq72@icineca.cineca.it (LTM/IOR)

Dear Dr. Dapena,

I saw a lecture some years ago in Miami (appropriately enough) about hard-
and soft tissue injuries from bullet wounds. I don't remember details, but
the lecturer spent significant time discussing the design of the bullet
casing in terms of physics equations for energy transfer (from the bullet
to tissue). In short, bullets are designed not necessarily for best flight,
but rather to maximize the damage they can cause by maximizing energy
transfer to tissue. I'm sorry I can't be more specific--perhaps you could
contact someone at the Department of Orthopedics and Rehabilitation,
University of Miami School of Medicine, Miami, Florida (Dr. Mark Brown,
Chair). Good luck.

Sincerely,

Marc Ramer
Rizzoli Orthopedic Institute
Bologna, Italy


---------------------------
Laboratorio di Tecnologia dei Materiali tel.
39-(0)51-6366864
Centro di Ricerca Codivilla-Putti fax.
39-(0)51-6366863
Istituti Ortopedici Rizzoli
E-mail:lk1boq72@icineca.cineca.it
---------------------------------------------------------------------------
from "William L. Siler, Ph.D."

Jesus:

I have no empirical evidence but have always believed that the flat end
provides for a situation in which the charge is compressed to as great a
degree as possible and the relationship between the charge and the bullet
is such that the force is applied perpendicular to the area of the bullet
exposed to the charge, providing the most effective application of the
force.
If that is correct, the velocity gained by the flat back may be expected to
be greater than the velocity loss to drag by the flat back.

William (Bill) Siler, Ph.D.
Saint Louis University
silerwl@sluvca.slu.edu
---------------------------------------------------------------------------
from Jian-Yu Cheng

Hi, Jesus,
Bullets may move at speeds higher than the sound speed.
In supersonic flow, the wave (shock) drag has the largest
contribution to the total drag. Streamlining the rear part
of the body, as used in subsonic flow, has little effect.

Jian-Yu Cheng
---------------------------------------------------------------------------
from "Peace be with you."

Jesus


Hopefully this will answer some of your bullet questions. Initally
bullets were round. Easier to cast I guess. They had pretty poor areo
performance. They they went to either the flat base or a hollowed base. The

hollowed base allowed the back of the bullet to expand and engage the
rifling
for more accurate flight. I would GUESS that bullet bases then became flat
to
make casting easier than the hollowed back. This would be around 1870's.
Today
the most accurate cartridges(used for the 1000yd matches) use boat tail
shaped
bullets. The back ends are angled for good areodynamics.


An interesting aside - a fellow who writes for one of the gun
magazines
did some testing with old blackpowder cartridge rifles(Sharps, Remington
RollingBlocks - the kind the buffalo hunters used in the mid to late
1800's). These
rifles were very accurate a long range(1000yds). From his testing he
discovered that the SD of the velocity of the rounds was less than 10
feet/sec. So the
accuracy came from the fact that the bullets were experiencing the same
forces
through the entire flight. Unless of course the wind changed, etc. He then
used
smokeless powder to discover that the SD was greater than 50-100fps. So he
calculated? that the bullets trajectory would be effected by several feet at

1000yds.


Just thought you'd like to know.

Dave

************************************************** ***************************
***
Dave Giurintano
Paul Brand Biomechanics Lab
GWL Hansen's Disease Center
Carville LA 70721 giurin@resdjg.gwlhdc.lsu.edu
---------------------------------------------------------------------------
from Kevin P Granata

Sir,
An associate of mine informs me there is an internet news group /
bulletin

board for guns and technical issues associated with fire-arms. They may be

able to answer your question, and would enjoy the discussion. This group
can

be found at rec.guns
Please do not reply to me, I am not familiar with this topic, just

relaying the information.
Good Luck
---------------------------------------------------------------------------
from "Gideon Ariel"

Hi Jesus:
The reason for flat bottom could be because after the powder
is ignited a minimum pressure must be built up inside the
bullet. So, the contact surface with the shell sould be
maximized. If the bullet is tapered, then some part of the
bullet will not be in contact with the sleeve, and will
cause lost of pressure.
Another reason could be that in the production line it is
easier to place the bullet in a streight line. A tappered
bullet could cause deviation from streight line.
Another reason could be that after the bullet is propelled
forward in barrel, unequal pressure could move the bullet
from side to side and cause inconsistency in the movement.
These are only opinions and not evidence.
Gideon Ariel
---------------------------------------------------------------------------
from keith@cvs.rochester.edu

SOunds like a good question for rec.guns. My guess is that is has something
to

do with getting an even distribution of pressure on the bullet on the way
down the barrel.

Keith Karn
keith@cvs.rochester.edu
---------------------------------------------------------------------------
The following is from a respondant that wished to remain anonymous ("I'd
just prefer to be known by the net community by other accomplishments than
facility with firearms."):

I think the answer to your question concerning the blunt rear-end of typical
lead

bullets has to do with the way the bullets are brought to speed, namely,
expanding

products of combustion. When the ammunition for a typical rifle is loaded,
the

bullet diameter is slightly smaller than the barrel diameter -- it has to be
to fit

into the breech. When the bullet is fired, the action of the expanding
gases on

the (slightly concave or flat) rear of the bullet expand the skirt of the
bullet

against the walls of the barrel, engaging the rifling grooves. This firm
contact

with these spiraling grooves (about one twist per barrel length) is
necessary to

spin the bullet up to speed (several thousand RPM). This spin gives rifle
bullets

greater stability and hence greater accuracy than bullets that are
immediately

allowed to tumble. Contrast this effect with a musket ball. A symmetrical
musket

ball, in a smooth barrel, need not expand to make contact with the barrel.
This

means that musket balls (and barrels) can be made to less restrictive
tolerances,

but also that they are unlikely to be as accurate as a rifle.


A bullet needs to have a sizeable amount of skirt (the cylindrical section)
to

engage the rifling. Typically, high-velocity, high-accuracy rounds are
narrower

and longer, and can be used with barrels having more rifling twists. A
bullet

could be encased in a carrier that engaged the rifling and then fell away,
allowing

a more symmetrical bullet to continue, and I believe that some are.
However,

without *any* real analysis, I'd offer the following thought: Given the very
large

amount of money that has been spent in the last century on armaments, the
fact that

small-arms bullets still have the same basic shape suggests to me that it
probably

must work pretty close to optimally.

---------------------------------------------------------------------------
from annaa@garnet.berkeley.edu (Anna Ahn)

Jesus,
I asked a friend of mine, who is not a biomechanist, b/c I knew he would
know.
Hope this helps,
Anna

>Date: Mon, 21 Nov 94 14:19:19 PST
>From: thuff@Csa1.LBL.Gov
>Subject: RE: bullets
>To: annaa@garnet.berkeley.edu
>X-ST-Vmsmail-To: ST%"annaa@garnet.berkeley.edu"
>Status: O
>
>
>Anna:
>
>Feel free to forward this answer to Jesus Dapena (go big red!!)
>
>The back of the bullet doesn't really have much to do with slowing on
>impact...that's a function on the point and whether or not it's a
>soft point and/or hollow point, etc.
>
>All the small arms bullets on the market do indeed have flat back ends
>and the reason is simply practical manufacturing. For al the copper
>jacketed bullets they start out with a form which is basically a cup
>for shaping the copper. Then the lead is poured inside the copper.
>They handle the bullet during the manufacturing from the a back end
>with a dye. To make a bullet that was perfect and tapered front and
>back would be difficult and time consuming because they would have
>to handle the tip more. In the manufacturing, they try to handle the
>tip as little as possible to keep from causing deformities which
>affect the accuracy. That's why hollow points are more expensive than
>non-hollow points.
>
>There is a type of bullet widely used in rifle for match competitions
>and for hunting called the full metal jacket boat tail (available in
>soft/hollow point for hunters). The "boat tail" is a tapered back end
>(tapered length is approx. 10% of the overall length) but it's
>flat after that. The turbulence behind even these boat tails is still
>significant and as a guess one loses perhaps 20% of a full-tapered-tail
>performance...if it was shaped and weighted perfectly.
>
>Regarding farther and faster... a very long hunting shot is 400 yds. and
>only the best shots would take that under optimum conditions. They have
>match competitions out beyond 1000 yds. I think so many other things
>become important at those distances that making a full-tapered-tail
>bullet isn't worth the expense. Perhaps someday as the rifles and other
>equipment improve and the shooters get better it will become important.
>After all, they used to have long range match competitions with
>round lead balls made by hand.
>
>I hope this answers the question sufficiently!
>
>tony
>thuff@csa.lbl.gov
---------------------------------------------------------------------------
from "Thomas G. Loebig"

Jesus:

If you are interested in aerodynamic efficiency, as in airplanes, cars, etc,
then a tapered bullet would be fine, but with a bullet, if you provide
enough
power to move it, who cares how efficient it is. Combustion is a sudden,
violent increase in pressure, a flat surface has the most exposed area for
the pressure to act on, F=p*A . . . brute force wins! But, if its fluid
mechanics explanations you want, here are my entries (which may combine with
p*A).

1. Don't immediately assume that the bullet does not deform following
detonation.
2. It is well know in aerodynamics that drag can create stability, and thus
increase accuracy.
3. However, that may not be the intention at all. Consider a golf ball.
At
first glance, the dimples on a golf ball may be suspected to cause added
drag
to the ball, but as we learn in undergrad fluids courses, the dimples add
energy to the boundary layer and counteract the effects of the adverse
pressure gradient on the rear of the ball. The result is delayed
separation,
a smaller wake, and reduced drag.
4. The speed is also important. I don't know for sure whether all guns
fire
at supersonic velocities, but that may be a factor also.
5. Last but not least, all guns are rifled, which imparts a spin to the
bullet, which in turn alters the flow, which also increases accuracy. Most
rifling is very slight and intended only to keep the bullet from tumbling
and
may not affect flow much at all.

Hey, this sounds like a good senior project.

Tom

Thomas G. Loebig, MSME Research Associate
Allegheny-Singer Research Institute tom@biomechanics.asri.edu
320 E. North Avenue,10th Floor ST voice: (412)359-6773
Pittsburgh, PA 15212-4772 fax: (412)359-3494
---------------------------------------------------------------------------
from "Peak Performance Tech."

Hi Jesus-

A few thoughts in regards to your bullet question:

Perhaps the shape of the rear of the bullet is related more to it's
propulsion than to it's drag characteristics. In the same way a tapered
front reduces the a resistive force (drag), a tapered rear would reduce
the propulsive force. In other words, a propulsive force would have a
greater effect on a flat end than on a tapered end. In fact, some
bullets actually have a slight concavity to the rear end, which would
result in an even greater propulsive force than a flat end.


*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
George Miller and John Porter
Peak Performance Technologies
Englewood, CO
76244.3047@CompuServe.com
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
---------------------------------------------------------------------------
from wheat@intermed.com (Wayne Wheatley)

This is not a mechanics question, but an aerodynamics question. Bullets
travel faster than the speed of sound. Therefore, the airflow over the
bullet does not enter as a factor in drag (until it becomes subsonic). What
is important is wave drag. As an object exceeds the speed of sound a 'wave'
is created behind the leading edge. The area behind this wave is a low
pressure area. The faster the bullet the larger the low pressure area (it
approaches 90 deg. to the flight path), thus the larger the wave drag. The
back of a bullet never 'sees' airflow in supersonic flight. Some bullets
are tapered at the trailing edge (boattail) to help it aerodynamically as it
slows from supersonic speeds.


As for the case. Most cases are not flat inside and not completely filled
with powder. The inside of the case is spherical at the primer end. Better
ignition is accomplished at about 60 - 75% powder capacity. This is because
the shell is horizontal when it is fired and the surface area of the powder
is greater when it is fired giving the primer more powder surface to ignite.

Wayne Wheatley
Intermedics Orthopedics
wheat@intermed.com
---------------------------------------------------------------------------
from "S.RASSOULIAN"

This is not a subject which I would like to think or talk about
on a Wed morning. When I decided to enter this field (i.e. Bioengineering)
I liked the idea of having to think about more constructive things, you know
like the things that may help in designing better aids for the disabled.

To answer your question I think the idea of having a bullet around is to
shoot it at somebody and inflict maximum damage, hence the flat end so
that the bullet either remains within the body or when it leaves it

creates maximum damage to the tissue.

sincerely
---------------------------------------------------------------------------
from "N.D. Barnett"

Hi there,
A bullet needs a flat end so that the pressure created in the case after

ignition is transmitted to the bullet in a way to produce forward motion

of the bullet. The force will act normal to the contour of the bullet,

hence a tapered bullet would result in forces acting radially inwards,

and not directly forwards. The result would therefore be a much slower

bullet.

FLAT BULLET
______
-- > | \
-- > | > ----->>> Motion
-- > |______/

TAPERED BULLET


`, ______
`> / \
-- > < > ----> Motion (slower, less force in this
direction)
,> \______/
,`


Hope this helps!

Nick Barnett.


__________________________________________________ ____________________
Nick Barnett.

Centre for Rehabilitation and Engineering Studies (CREST),

University of Newcastle-Upon-Tyne,

Newcastle-Upon-Tyne, e-mail ......: N.D.Barnett@ncl.ac.uk
England. Telephone ...: (+44) 0191 222 6196

---------------------------------------------------------------------------
from Ulrich Glitsch

Dear Jesus,

At first I am truely not a specialist in shooting, but I think
there are some simple explanations which may help.

In the gun's tube it is necessary to transfer the energy from
the explosion at ignition to kinetics energy of the bullet. I don't
know if there is a drawback with tapered bullets, but I assume so (you
mentioned this in your posting already).

Most important in shooting is reliability of the bullet's trajectory. This
has to do something with stability of the movement, the resistance

against disturbances (air forces). In relation to this the range is
of much less importance.


Obviously there will occur a lot of turbulences at the back of the

flat bullet causing a great drag force at this part. This tends to improve

the stability besides the spin of the bullet. This is similar to the

fethers at the end of Robin Hood's arrows. Robin Hood would miss his
object absolutely if you take away these fethers. Also the range may
be less because the arrow hits the ground during its unstable flight.




Best regards
Ulli

--

------------------------------------------------------
!Dr. Ulrich Glitsch !
!Institut fuer Biomechanik !
!Deutsche Sporthochschule Koeln !
!Postfach !
!50927 Koeln !
!EMail: glitsch@iris.dshs.uni-koeln.de !
!Phone: +49 (0221)/4982-567 !
!FAX: +49 (0221)/4971598 !
------------------------------------------------------
---------------------------------------------------------------------------
from KUNZ@ORTHO.SURGERY.WISC.EDU

Dear Jesus;

You are correct in suggesting that a long tapered tail would decrease

drag on a bullet. I remember something about 12-15 deg. as

an approximate angle. Some companies make "boattail" bullets with a

truncated angle at the base that reduces drag enough to significantly

affect trajectory at ranges over 100 yards.


Other important considerations however, limit the amount of boattail

that is feasible. Bullet length and weight, a constant diameter area

sufficient to align and stabilize the bullet in the brass case and to

engage the rifling grooves in the barrel, erosion of the barrel

immediatly ahead of the chamber apparently due to the shape of the

combustion chamber, and the impact performance of the bullet when it

hits the target in hunting situations are some of these factors.

Manufacturing considerations present another list of trade-offs to

consider. I'm sure that there are others.

David N. Kunz
Kunz@ortho.surgery.wisc.edu
U of Wisconsin, Madison
---------------------------------------------------------------------------
from diro@sirius.medizin.uni-ulm.de (Dieter Rosenbaum)

Jesus:

I do not know much about shooting and the aerodynamics of bullets
but at first sight it appears to me that a flat back might be

beneficial during the propulsion phase after ignition while the bullet

is still in the barrel of the weapon. It may "offer" a flat surface

in order to minimize losses of the ignition force. Therefore, even

though the flat-end bullet design does not make sense from the

aerodynamical point of view during the flight phase, from my

non-expert point of view it makes sense for the propulsion phase.

Can you buy that explanation?

Ciao,

Dieter...


************************************************** ***************************
***
* Dr. Dieter Rosenbaum voice0)731 - 502 3492
*
* Abteilung Unfallchirurgische fax: (0)731 - 502 3498
*
* Forschung und Biomechanik email: diro@sirius.medizin.uni-ulm.de
*
* Universitaet Ulm
*
* Helmholtzstr. 14
*
* 89081 ULM
*
* Germany
*
************************************************** ***************************
***
---------------------------------------------------------------------------
from Djdalton@aol.com

Hello Dr. Dapena,

Three possible reasons (from most likely to least likely) that the rear end
of a bullet is flat:
(1) I would think that there would be more force against the bullet with a
flat surface.
(2) Perhaps when the powder fires the bullet is more likely to go straight
if
the surface area on the back of the bullet is large and flat; if it were
tapered it might be skewed upon ignition.

(3) Maybe as the bullet moves through the chamber the amount of surface area
in contact with the barrel, or the shape of the bullet as it travels through
the barrel, has an effect on the rotational characteristics of the bullet,
which would affect the flight. As you know, a spinning projectile will go
straighter and be less apt to be influenced by disturbance than one that is
not spinning.

You're right that from an aerodynamics standpoint, there would be less drag
if the bullet did not have a flat back side. The sharp corner causes a
tremendous amount of drag.

I'm interested to see other replies.
Donna Dalton, MS
Instructor of Biomechanics
Cal State Univ, Long Beach
djdalton@aol.com

---------------------------------------------------------------------------
from SALO_AIT@west-london-institute.ac.uk

Dear Dr. Dapena

Your bullet question is very interesting.
However, I would like to go even further in
creating this hypothetical bullet -


why could the bullet not have dimples as

golf balls have?

Aki Salo
West London Institute
UK
---------------------------------------------------------------------------

That's all, folks. Now, on to BIOmechanics!
---
Jesus Dapena
Department of Kinesiology
Indiana University
Bloomington, IN 47405, USA
1-812-855-8407 (office phone)
dapena@valeri.hper.indiana.edu (email)