From: Louis Boyd <boyd@pegasus.la.asu.edu>
Subject: Re: Football Shaped Bullets?
Date: 10 Jan 1997
Newsgroups: rec.guns

amo!michael@uunet.uu.net (Michael Courtney) wrote:
#Ballistic coefficient is a key in long range shooting because reducing
#drag reduces bullet drop.  As a result, long shots are easier to make
#without exactly knowing the range.  Wind effects are also reduced.
#Current boat-tail spire-point bullet designs offer the highest ballistic
#coefficients on the market.  Why not take the boat-tail concept to its
#logical extreme and bring the back of the bullet to a point to increase
#BC?  This would give a football shape to the bullet, although the
#shape for optimum BC might not be symmetric about the midpoint.
#How big an increase in BC is possible by reshaping the bullet this way?
#
#Thre are some possible disadvantages I have thought of:
#
#1. Angular momentum is decreased for the same bullet weight, rate of twist
#and muzzle velocity, because bringing more mass toward the bullet axis
#reduces the moment of inertia.  This will slightly destabilize the bullet,
#but it is probably a small effect and could be compensated for by larger
#bullet weight, higher twist rates, or greater muzzle velocity.
#
#2. Bullet performance could suffer.  Designing a football shaped bullet
#with similar performance to today's premium bullets would be difficult.
#
#3. Powder volume would be reduced because the point at the back of the
#bullet would be in the cartridge.  Again, this is a small effect, but the
#people who care about performance enough to use these bullets might
#already be using compressed loads.
#
#Do the technical difficulties in overcoming these and other possible
#disadvantages outweigh the performance gain in BC offered by the
#football shape? Both expert and novice comments are welcome.

Obviously there are a lot of factors which go in to the design of a bullet,
and lots of various shapes have been tried. They've been tried in firearms
as well as wind tunnels and computer simulations.  What is used is what works.
To achieve a high BC, obviously drag must be reduced.  Drag is mostly
determined by the angles of the front of the bullet (ogive).  The turbulance
and "vacuum" behind the bullet have some significance but not nearly as much as
the ogive.  If you made a bullet "football shaped", it would have to be less
tapered in the front for the same diameter and weight and would therefore
have a lower BC.

It's easy to design a higher BC than in conventional bullets.  Instead of
using a copper jacketed lead core just use a denser core.  Metals
more dense than lead include (but not limited to):
gold                 expensive
depleted uranium     hard to obtain, hard to machine
tungsten             very difficult to machine. Practical for some
applications.
osmium               expensive
iridium              very expensive
platinum & palladium very expensive
Most other other metals denser than lead are very rare and increadibly
expensive.

There have been some exotic bullet designs which have had (very) limited
success.
A "wasp waisted" bullet which was produced by Herters Corp some years ago.
They claimed excellent accuracy, but test proved other designs to be
more accurate. Hollow cylindrical bullets (like a pipe) exist.  They maximize
rotational inertia for a given weight and have low sectonal density. A light
weight "plug" falls away when the pressure drops behind the bullet as it exits
the muzzle.  This round get's reinvented occasionally with claims of high BC's,
but notice they don't appear commercially.

Another point of bullet design is that it must be stable when there is very
high pressure behind the bullet and nothing holding it.  This occurs just
as the bullet passes the muzzle. It is the time when the highest upsetting
(tilting) forces are on the bullet.  A long point sticking into the barrel
as 5000-10000 psi gasses blast past it won't do anything good for accuracy.

Yet another requirement is for the bullet to provide good bearing and
engraving surfaces as well as a good seal as it is forced through the
barrel.  Notice that most benchrest bullets have a flat base.  This allows
long bearing surfaces and a quick transition as the bullet passes the crown.
You can be sure that benchrest shooters know what works.  They sacrifice
some drag for minimal upset as the bullet exits.

For long range shooting a high ballistic coefficent becomes more important than
the slight additional upset error caused by having a boat-tail design but
balance and strength are very important too.  In the 1000 yard .50 caliber
matches you primarily see very long solid steel or solid bronze bullets
winning the matches. These are generally machined, not cast or swaged, to
very tight dimensional tolerances.  This is expensive. Not as expensive
however as if the bullets were made of exotic dense metals as discussed above.
While ogive shapes vary slightly, essentially all are spire-point boat-tails.

Lou Boyd
FCSA  NRA