This page documents some of the
thinking and research that I have been undertaking in order to more
fully understand the physics of a rifle during the actual shot event.
I started thinking about why some
rifles shoot better than others, why small changes in load parameters
can make such a large difference in accuracy, and why "good" loads seem
to work pretty well in a large number of different rifles, even rifles
with very different barrel profiles and stock configurations.
The theory presented here, what I
have been calling Acoustic Shock Wave Theory, leading to the concept of
Optimal Barrel Time (OBT), is my attempt at answering these questions.
The concept is simply that there is a very deterministic physical
process that causes a rifle bullet to diverge from the statistical mean
by a statistically greater or lesser amount, depending on the time of
bullet barrel exit in relation to the time of initiation of the
propellant burn event.
This hypothesis differs from most
vibratory barrel models in that it is based on the proposed existence of
longitudinal shock or acoustic waves in the barrel steel, and not simple
harmonic bending vibration modes. Prediction of these modes is
relatively simple compared to the prediction of the bending modes,
and is based exclusively on the length of the barrel, and not the
contour or thickness.
The rifle barrel, stock, bullet,
and propellant is considered as a system, and some of the possible
interactions are explored. Computer simulations were performed to model
the longitudinal wave shapes. These wave shapes led to the discovery of
the presence of optimal exit times for the bullet: times where the
muzzle is the least disturbed in a statistical manner, and therefore the
least likely to perturb the bullet at launch. Tables of these times were
generated, and a simple linear system of equations developed to allow
easy computation of these times.
Extensive range testing using
QuickLoad, a chronograph, and a PressureTrace instrument was performed,
validating the presence and location of these exit times.
I have, since the paper was first
made public, received many hundreds of independent confirmations of the OBT
values by shooters at large. The physics may not be
explained in a rigorous manner, but there is now far too much empirical
evidence to doubt that there exists some process that has the periodic
patterns that my original research has predicted. As a practicing
engineer, I can accept this fact without having formal proof. If it
works, use it...
The paper presented here is my
first writing of the concept, and has not been significantly edited to
correct tortured syntax, typographical or grammatical errors, or to make
it more readable. You are seeing my thoughts as I wrote the paper one
long afternoon. I make no apologies for the less-than-polished format.
I encourage you to read the
material presented here, and try this out for yourself. If you have a
few pet loads, and access to QuickLoad, use
QuickLoad to determine the
barrel exit time for these loads, and compare these times to those
predicted by this theory. And, if you have the time, please drop me a
line to let me know what you observe.