WHAT IS AN OPTIMAL CHARGE WEIGHT
With any given bullet and powder combination, there
will be a specific amount of that powder which will
cause the bullet to exit the muzzle at the "friendliest"
portion of the vibration cycle. This does not necessarily
correspond with the tightest velocity figures, however.
Many load recipes have over the years become the "go
to" recipes for trying out a new rifle. If your rifle fails to
shoot one of these "tried and true" recipes well, you may
well have a rifle integrity problem.
Consider the Federal Gold Medal Match .308 win ammo.
How can one recipe shoot so well in so many rifles?
That's what we're after in the OCW load--a load recipe so
stable and predictable that it does indeed rise above,
and to some extent disprove the "all rifles are different" adage of lore...
Think about it this way. Some kids like mashed potatos, some don't. Some kids will eat thier peas, some won't. Some kids like bananas, some hate 'em. But show me a kid that doesn't like chocolate ice cream and I'll show you one bizarre little character! :) An OCW load is like chocolate ice cream to your rifle. If your rifle doesn't like the recipe, there is likely something weird about your rifle.
If you perform the OCW load development properly,
you are not (in the initial stages) seeking the optimal
load for the test rifle. Heresy? Not really. There are
many known "universal recipes" that work well in most
rifles chambered for them. Ken Waters has made a pretty
penny with his book "Pet Loads" on this concept. Many
of the Waters loads have worked beautifully in my rifles,
and acquaintances have had the same experience.
Federal's Gold Medal Match .308 loads do well in
practically any .308 not needing a gunsmith's attention.
How can this be? If you subscribe to the philosophy that
"all rifles are different," and therefore need individual
loads tailored to accommodate their idiosyncrasies, how
do you explain the near universal MOA performance of
loads such as the Federal GMM? Universally good
recipes do exist. Upon realizing this, my questions have
been "How do such recipes work?" and "How can I
develop such a load?"
Recent conversations with very learned mechanical
engineers are affording me some better understanding of
just why an OCW load works so well in the majority of
rifles chambered for the cartridge at hand.
Here are some of the major points coming to light:
Uniformity of velocity (meaning low extreme spreads of
velocity) are definitely not an indicator of the OCW
zone. We are actually finding that in many cases the
OCW zone does not have the tightest numbers--at least
initially. Fine tuning of the recipe with seating depth
variations and primer changes will improve the velocity
consistency, but simply shooting a succession of
graduated charges over the chronograph and looking for
a tight velocity spread will not lead you to the OCW.
As the reader comes to understand more about the main
shock wave (link Chris Long's pages elsewhere in this
webpage) it will become easier to understand how
bullets with larger extreme spreads in velocity can still
group tight (at closer ranges), while bullets with
seemingly tiny extreme spreads may group poorly. You
can tighten the extreme spread to improve long range
accuracy AFTER you identify the OCW.
Engineer Chris Long's model of barrel
behavior suggests (simply put, and in part)
that the initial shock wave, generated by the powder
charge's ignition, travels at the speed of sound in
steel (about 18.000 fps) from the chamber to the muzzle, then back, in a repeated pattern. When this wave is present at the muzzle, there is
naturally much turbulence and obturation of the
"roundness" of the bore at the muzzle. However, when
this main shock wave has reverberated back to the
chamber end, the muzzle is relatively stable. This
window of opportunity, according to Chris, is the best
time for the bullet to exit the muzzle. The barrel is
basically straight, and relatively calm.
(Read later about the "scatter node" which is the point
at which the bullets are being released from the bore
when the shock wave is at the muzzle. I call this area
the "scatter node" because it will produce a scattered group,
throwing flyers at random. This is the most
inopportune point of all for bullet release, the scatter node area
can be easily seen during an OCW test, and generally 1 to 2
powder graduations above the scatter node charge will have
you right in the OCW zone. The existence of the scatter node
is the main reason the conventional ladder (Audette) test often fails
to yield useful results--see OCW vs. Ladder on this site).
The OCW load would then have the optimal amount of
powder to push the bullet at just the right speed to be
exiting the muzzle when this shock wave is at the other
end (the chamber end) of the barrel.
This shock wave travels at the same speed regardless of
barrel length or girth (about 18,000 fps).
We note that OCW loads seem to perform very well
regardless of barrel length. (We're not considering non-
typical barrel lengths here). This makes sense because
the relationship between barrel time (the length of time it
takes the bullet to exit the muzzle) and the oscillating
shock wave is close to the same; in other words, in a short
barrel the shock wave reaches the muzzle and returns to
the chamber faster, but alas, the bullet reaches and exits
the muzzle faster also.
The harmonics which ride the main vibration node are
only of minor importance to accuracy in most cases. It
would appear that the effects of the subtle harmonic
vibrations can be largely negated with seating depth
adjustments, but a true OCW load will normally be
MOA or better in a good rifle without seating depth