From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Newsgroups: rec.guns
Subject: Re: Bullet Construction and Performance
Date: 10 Apr 1994 14:44:42 -0400

Doug Lynch (drl00@lvld.hp.com) wrote:

: So I've been thinking about a few questions for you netters:

: 1)  How much affect does the tip of the bullet have?  Match bullets are
:     typically hollow points.  Wouldn't the Nosler BT "perfect point" have
:     more accuracy potential than a hollow point or a lead tip that always
:     seems to have some dents and imperfections in it?

The most accurate parts of a bullet are in two places:

  * Bearing surfact to base junction uniformity.  Hollow point bullets
    are able to be made more uniform in this critical area.  As that's
    the last part of the bullet that leaves the muzzle, any irregularity
    at this junctiol will cause the bullet to tip as it exits.

  * Jacket wall thickness uniformity.  As this determines how the bullet
    is balanced, any non-uniformity in wall thickness will put its lead
    core offset from its center of form.  When spun at hundreds of 
    thousands of RPMs, any offset the center of gravity has from its
    center of form will cause the bullet to wobble just like a wheel that
    is out of balance.

: 2)  Hornady builds their bullets with what they call a secant ogive shape.
:     What really is this, and how does it compare to other bullet shapes?

A secant ogive is a raduis based on a trig function.  Most bullets have
their ogive a true arc whose radius is typically measured in calibers, or
bullet diameters.  

: 3)  What about boattails versus flatbase bullets?  What are the pros and
:     cons of each?

Boattail bullets typically have flatter trajectories as their drag, or
ballistic coefficient is higher.  But that advantage is not worth much for
ranges less than 500 yards because 99.999% of the groups folks and their
rifles shoot at longer ranges are larger than the higher BC advantage gets
them.

Commercial bullet companies have found that in factory barrels, flat-base
bullets may well shoot more accuratly than boattail bullets.  In true
match-grade barrels, boattails do the best.

: 4)  Some bullet makers manufacture their bullets with oversize tolerances.
:     For instance, Hornady 30 cal bullets measure approx .308 while Lapua 
:     bullets can be had measuring .309.
:     What real difference will oversized bullets provide?

Virtually every rifle barrel made shoots the most accurate when the bullet
diameter is at least five tenthousandths of an inch bigger than the bore's
groove diameter.  In fact, bullet diameter can be as much as .0015-in. or
more bigger than groove diameter and peak accuracy still exists.  One
advantage of shooting bullets bigger than groove diameter is the barrel
will last somewhat longer before accuracy starts to decline.

: 5)  What about the internal structure of the bullet?  How about the way
:     the copper was drawn and does the type of lead used make a difference?

Jackets are first coined from a sheet of jacket material.  Coining is just
stamping out round pieces.  These `coins' are then put through several cup
and draw processes that makes the tube of jacket material with one end
closed.  Next, the open end is trimmed to length.  It's the wall thickness
of these jackets that is critical for accurate bullets.  Match-grade
jackets vary only a few ten-thousandths in wall thickness; hunting bullets
about 2 to three times as much.

Lead cores may be pure or hardened lead.  A large chunk of lead a few inches
in diameter is pressed in a swage that squirts out lead wire of the right
diameter.  That wire core is next cut to the desired length.  A machine
presses the core into the jacket.  Finally, a pointing machine presses the
cored jacket into a die that shapes the bullet.

Pure lead cores are very homogeneous and therefore are used in match bullets.
Hardened lead cores are used for many hunting bullets so they won't expand
so fast.  But hardened lead cores are not too homegenous; sometimes they
put the center of balance offset from the bullet's center of gravity.  Which
is why hunting bullets aren't as accurate as match bullets.

: 6)  How is ballistic coefficient calculated and how important is it in
:     the performance characteristics?

Ballistic coefficients are calculated by time of flight between two points
that have chronographs.  Sierra's Handloading Manual has about the best
explanation of this.  Bullets that take less time to go from point A to
point B with the same velocity at point A have a higher BC than those that
take longer.

BC only tells us how fast a bullet's velocity will drop as it goes down
range.  BC, by itself, has nothing to do with accuracy.  But when coupled
with velocity, higher BC bullets typically have less vertical stringing at
longer ranges.

BB