Is
One Powder more Accurate than another, and other various Topics
. . .
There
seems to be a popular misconception that, in the Savage 10ML-II
application, such a thing as "a more accurate powder"
exists. It does not.
There are propellant
characteristics that may make a powder seem more accurate, causing
folks to say "my gun does not like xxx powder,"
but that really is not possible. Accuracy and consistency are interchangeable,
so naturally a propellant that is not consistent lot to lot can
gain a reputation as being inaccurate. That has been the observed
case with Hodgdon-IMR SR4759, a very good propellant for 250 grain
saboted bullets. However, you can gain or lose 100 fps in short
order when you open a can from a different lot. It might seem "inaccurate,"
but we just need to adjust our load up or down a couple of grains
to find the same sweet spot of muzzle velocity that gave us good
results before.
A Muzzleloader is a System
Richard Lee touches
on a couple of interesting points in his Modern Reloading, 2nd
Edition, pp. 124-125. In his "velocity & pressure factors"
for a 1 % percent charge reduction, Mr. Lee notes that while a 1%
charge reduction usually reduces velocity LESS than 1%, that same
1% charge reduction reduces pressure by more than 250% of the minimal
(or virtually indistinguishable) loss of velocity. We are
dealing with the cumulative effects of tolerances in muzzleloading:
rifle to rifle, vent-liner, sabot, bullet, primer along with divergent
ambient shooting conditions not the least of which is ambient temperature.
To cite a specific example, a 47 grain charge of Accurate Arms 5744
pushing a 300 grain XTP may be sufficient to damage a sabot beyond
its ability to function accurately in several Savage 10ML-II's.
Use of 43 or 44 grains of Accurate Arms 5744 gives us far more reliability
and accuracy cushion than our chronograph would indicate-- it is
a load proven reliable and accurate in most Savage 10ML-II's. Less
can mean more, and in this case I believe it is-- more reliability,
more repeatable accuracy, far less likelihood of exceeding sabot
limitations regardless of temperature extremes. Where 47 grains
of 5744 may frustrate; 43 grains of 5744 is more likely to make
you smile.
Here's look at how flexible
5744 is, showing results caused by an uncharacteristically huge
± 10% powder lot-to-lot burning rate variation using a nominal
44 grain charge.
Data
for burning rate increased by 10% relative to nominal value:
+Ba 44.00 gr. 2264 fps.
Data for burning rate decreased by 10% relative to nominal value:
-Ba 100 44.00 gr. 2007fps.
Despite this twenty
percent swing in projected burning variation, our muzzle velocities
cannot vary nearly that much. The only other Savage compatible
powder that is this insensitive to burning rate changes is Alliant
2400, a faster and more limited use propellant. This is part of
the reason I believe Accurate Arms 5744 to be the most temperature-insensitive
10ML-II propellant you can use.
Accuracy of a given
bullet is contingent on our ability to stabilize it by imparting
spin to it. Only our barrel's rate of twist and muzzle velocity
determine that. You cannot "over-stabilize a bullet."
We seek to make our bullets as stable in flight as possible, therefore
saying that they somehow be can be too stable is like saying our
gun can be "over-reliable" or "over-accurate."
There just is no such thing. Whatever propellant that gives us equal
muzzle velocity without compromising sabot performance will give
equal accuracy in the Savage 10ML-II.
Heat of Combustion
Classic black powder
has a combustion temperature of about 3000 degrees F., "grey"
powder (with elevated sulfur and potassium nitrate levels)
burns at about 4000 degrees F. Empirical data for gluconic acid
propellant (Triple 7) burn temperature has not been released,
but appears to be somewhat higher-perhaps in the 5000 degree range.
Accurate Arms 5744 has a combustion temperature of 5850 degrees
F. Other smokeless powders burn in the same general area. At first
blush, it would appear that this elevated temperature would make
it tough for sabot survival, but that is not the case. Part of the
reason is the brisance of smokeless powder.
Peak Pressure
Brisance in propellants
is just a way of measuring speed with which a powder develops its
peak pressure. It might seem that AA 5744 burns too hot for a sabot,
but we know that isn't the case by experience. What happens is that
is takes only about .4 milliseconds for a 44 gr. charge of 5744
pushing a 300 gr. bullet to hit peak pressure, and it drops off
rapidly from there. By the time about .6 milliseconds have elapsed,
pressure has dropped off by about 15,000 PSI. Our sabot is blasted
with a bit more heat, but the peak pressure of that gas is momentary,
and drops off rapidly after that peak node. The sabot takes a blast
of significant pressure, but it just does not last very long. The
point of high pressure is always behind the sabot, and as our combustion
chamber ends up as one extremely long straight case (barrel length)
the combustion chamber size is ever increasing in concert with sabot
base movement.
How Powder Burns
In the literature, you'll
often read a statement such as "black powder is surface
burning propellant." So far, so what? All smokeless
powders are surface burning, all small arms and muzzleloading propellants
are surface burning, so what this jewel of non-information has to
do with anything remains a mystery. You'll also read and re-read
that "smokeless powder is a progressive burning propellant."
Not really. Smokeless powder is both progressive and regressive
burning; interior ballistics calculations depend on the precise
values found in a specific propellant. A powder in a given application
is progressive as the surface area of a kernel of powder increases.
It is up to that point that its gas output increases. After the
maximum surface area point is reached, it becomes regressive for
the remainder of the barrel time-- producing less gas in concert
with the ever-diminishing surface area.
Why Isn't There An Accurate Powder?
Back to the original
topic, that being of thinking that one powder is more accurate than
another. There is no basis for that, excepting ancillary considerations
such as the amount and type of bore fouling left by a particular
propellant. The only other consideration is not strictly accuracy
related, but if we need to jump to a velocity not obtainable for
stabilization of a bullet (or a higher bullet weight) not allowed
by 5744, slower powders like Vihtavouri N120 for speed and Accurate
Arms 2015 for heavier bullets might be the better choice.
As
it turns out, Accurate Arms 5744 remains the only powder most Savage
10ML-II shooters will ever need, or could ever want. Not because
it is "more accurate," however.
Postscript: Johan Loubser on the "Launching
of a Bullet"
Johan Loubser of Western
Powders -- Miles City Montana, graciously consented to proof my
article, though he may more often be found addressing another form
of "proof." Johan Loubser is widely regarded as one of
the top three senior ballisticians in the world today.
Johan writes:
"From
an internal ballistic standpoint, which is the area where the shooter/reloader
has the most influence in the process, and which constitutes the
"launching phase" up until the projectile leaves the muzzle,
of a tube weapon, the accuracy is totally dependant on the specific
combination, and it is therefore impossible to theoretically predict
the result.
The
dispersion of the bullet is determined by the stability of the launch
platform/system, which off course includes the shooter. This stability
is controlled, and determined, by a complex interaction of the different
components primer case bullet and gun (as well as shooter) none
of which can be considered "accurate" as a separate entity,
for any specific combination. It's is not correct, and a common
misconception, to assume that there are so called "accurate-
powders, primers, cases, projectiles and weapons. Any one of these
claims can be easily disproved with the wrong combination. This
combination of primer, cartridge-case, powder, projectile/bullet
and internal chamber/barrel dimensions will deliver a particular
pressure impulse or signature.
These
components aspects/parameters all contribute to the "launch-impulse",
and performance level (velocity and pressure). The dynamic characteristics
of this impulse are therefore very specific for each combination.
How the launch platform/Rifle reacts to this impulse, determines
the stability, and as a result the accuracy/dispersion. This specific
reaction of the launch platform can be described the "dynamic
character or -finger print" of the weapon/ammunition system.
That's why an "accurate load/combination" can only be
achieved through incremental adjustments, and/or variations of these
different components, and parameters.
The only way to properly do this is, to always start at the minimum
or start load, and work up towards the maximum suggested load, with
the components at one's disposal. If the desired accuracy is not
achieved, the second phase will be to adjust or change the components,
i.e. primer, case, case condition, projectile/bullet or the type
of powder altogether.
The
important thing to remember is to change/vary only one parameter/component
at a time."
