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By Wayne McLerran
Posted 10/8/20

The fundamental process of melting lead into round balls to be used as
a projectile in the first smooth bore firearms has been around for
centuries.  More recently bullet casting has been the subject of many
articles, papers and books published by experienced shooters and
companies manufacturing casting equipment, and numerous articles
are available on the Internet.  One good Internet source is
//www.longrangebpcr.com/8Phases.htm in which Darryl Hedges
covers his 8-phase casting cycle.  While reading Darryl’s comments
keep in mind that the information is around 10 years old.  Since then
some new technology has been adapted to control and monitor casting-
pot temperature.  Also understand that, although I have been casting
bullets for several decades, I don’t consider myself an expert on the
subject.  But in a batch of 60 bullets the typical variance from the
average weight has been relatively small, varying +/- 0.5gr on a good
day up to +/- 1.0gr on a bad day of casting.  With the above
understanding I’d like to pass along what works for me.

But before proceeding I want to make it clear this article assumes dip
or ladle casting relatively heavy black powder cartridge rifle (BPCR)
bullets in the range of 350 to 550grs.  The mould is filled by
transferring the alloy from the pot to the mould with a ladle (aka a
dipper).  The majority of BPCR casters use a ladle which has proven to
produce better quality bullets with less weight dispersion when
compared to bullets from a mould filled from the spout of a bottom-
pour pot.  I started BPCR casting with an RCBS bottom-pour pot that
I'd used for many years for handgun bullets, but quickly realized the
advantage of ladle casting and used a ladle with the pot until
purchasing a modern digitally controlled RCBS Easy Melt “ladle pot”.  
Both pots are displayed in Figure 1.  The older RCBS bottom-pour pot
with a separate digital controller is in the background.
So you’ve finished casting a batch of heavy BPCR cast bullets and
decided to weigh some or all to determine the results.  After careful
weighing you find that the weight spread was larger than you
expected or prefer.  You’re concerned since you’ve likely read that
some top shooters have suggested that +/- 0.5grs is necessary or at
least the goal for match grade ammo, which may be unrealistic for
many casters depending on their technique and equipment.  
Personally I don’t know what variation in weight is necessary to
significantly affect BPCR accuracy but I’ve started an experiment on
the subject and hopefully will soon find out.  The results will be
reported in a future article.  Until then I plan to continue weighing
and sorting bullets into small weight groups for matches.  But let’s
get back to the subject at hand.

So what can cause large weight variations in bullets cast in a single
session?  Assuming a good quality mould that closes properly, there
are several factors that can affect bullet weights.  I’ll discuss some of
the others but the one that’s more important and “stands out” above
all the others combined is temperature, temperature of the mould
and temperature of the alloy, and mould temperature is the most
important of the two, hence my “single session” comment above.  By
the way, my definition of a single session is one that starts with the
first bullet cast and ends without any breaks in between.  I.e., a
session stops as soon as the process is interrupted or the alloy
temperature is affected for any reason: fluxing, spending many, many
seconds removing a stubborn sprue or stuck bullet, adding new alloy
to the pot, etc.

Weights can vary in visually perfect bullets due to a well understood
metallurgical process.  As the mould is filled with the hot alloy the
metal in direct contact with the mould cools first.  Since the center is
the last to solidify, a center shrinkage void is created.  Temperature
of the alloy and the mould affects the size of the shrinkage void.  The
dispersion in the size of the void is the main reason for the dispersion
in bullet weights.  If you’ve been casting any time at all you no doubt
have noted that as the alloy quickly cools in the mould a small dip or
cavity will form in the center of the sprue plate hole.  This is the
result of the shrinkage void and is the reason one should leave a good
size puddle of metal on top of the sprue plate.  The puddle supplies
additional alloy to help fill the cavity as the alloy cools.  Once the
puddle cools (freezes) a small shallow dimple will be located in the
puddle over the sprue plate hole, another indication of lead being
drawn in to fill the shrinkage void.

Now let’s consider what happens when starting a casting session with
a cold mould.  Due to the temperature differential it’s likely the
mould will not fill out properly resulting in visually defective bullets.  
But even if the bullets pass a visual inspection, until the mould comes
up to temperature the greater heat disparity will result in larger
shrinkage voids.  A way to eliminate tossing a bunch of bullets is to
preheat the mould.  I’ve experimented with and use a small
adjustable laboratory hot plate and have determined that preheating
a single cavity cast iron mould to around 450 degrees works for me.  
While preheating, a digital meter is used with a thermocouple probe
inserted into the mould cavity for monitoring the temperature.  Look
closely at Figure 1 and you’ll see the mould setting on the hotplate
with the probe inserted in the sprue plate hole.  Another method is to
apply some Tempilaq, a temperature indicating fluid to the outside of
the mould.  See Figure 2.  Tempilaq can be purchased in several
temperature ranges.  I suggest 450 degrees F is perfect.  It’s available
from Brownells, Amazon and other sources.  Those of you using an
aluminum or brass mould will have to experiment a bit to determine
the ideal preheat temperature.  By the way, having a hot plate is real
handy for keeping the mould hot during necessary session breaks.
Other factors that can affect bullet weights include: casting cadence,
ladle-to-mould head pressure, furnace or pot temperature control,
temperature stratification of the alloy in the pot, loose sprue plate
and lead deposits on the mating surfaces of the mould that prevent it
from fully closing.  By the way, I cut the sprue over the pot and allow
the cutoff portion to drop into the alloy.  The temperature effect of
dropping the small hot sprue cutoff into the much greater volume of
the hotter alloy is insignificant.  Others concerned that the cutoffs
will significantly affect alloy temperature collect and add them to the
pot after finishing a session.

Maintaining a constant cadence helps to minimize variations.  Once
you’ve figure out the correct timing that allows the sprue to freeze,
cut the sprue and drop the bullet, keeping the same time differential
between bullets helps maintain the mould at a constant
temperature.  If you’re using a bottom pour ladle such as the RCBS or
Lyman, when mating and rotating the ladle and the mould, the
amount of lead in the ladle determines the “head pressure” of the
alloy being forced into the mould.  More lead in the ladle increases
the pressure and minimizes the shrinkage voids.  So fill the ladle to
the same level each time.

Speaking of ladles, there are many styles but all are either bottom
pour or side-pour designs.  Figure 3 displays several examples.  Some
are very effective at separating dross f
rom the alloy being poured
into the mould, others are not.  The Lee side-pour ladle and similar
basic spoon designs do not separate the dross.  The Rowell style
comes in several dipper sizes.  It’s called a bottom pour but is
actually a side-pour design that captures the dross-free lead from the
bottom of the ladle and feeds it through a tunnel to one side.  The
spoon and Rowell styles do not create any significant “head
pressure”.  Having struggled casting acceptable quality bullets with
both the Lee and Rowel-style ladles I gave up on both for a bottom
pour design.  Although I use an RCBS ladle the Lyman ladle is very
similar and should work equally as well.  When the ladle and mould
are turned after mating, the bottom pour ensures the alloy does not
contain dross since it floats on top.  The unique design that I like
about the RCBS ladle is the long flange along the back that’s
convenient for moving the dross in the pot out of the way prior to
dipping the ladle.  The one negative of the RCBS and Lyman design is
limited capacity if you’re dip casting for a multiple-cavity mould.
Concerning casting pot temperature control, a couple of experiments
have convinced me that varying the pot temperature a few degrees
will affect bullet weights.  Since my pots have digital controllers the
temperature is easy to adjust by small amounts.  In two separate
experiments the temperature was changed by 5 degrees when casting
two styles of .40 caliber (400gr and 415gr) bullets.  The average
weight change was 0.7grs in one experiment and 1.0gr in the other.  
Hence one reason I use digital controlled pots and the reasons RCBS
and Lyman now sell casting pots with built-in digital controllers.  The
controller allows changing the temperature in one degree increments
and the digital display indicates the alloy temperature.  Do you need
a digital controlled casting pot?  Maybe not, but if you’re considering
one see my articles on the subject at the following links:

If undisturbed, the alloy temperature can stratify with the hotter
alloy rising to the top.  Or if the heater is on the alloy in the bottom
may be hotter.  Temperature stratification is less likely a concern
with digital controlled pots than when using a cast iron pot over a
propane burner.  Regardless, to ensure a constant alloy temperature,
I continue to mix the alloy by making it a practice of submerging the
ladle to the bottom of the pot each time it’s filled.

An improperly adjusted (usually too loose) sprue plate can affect the
final bullet weight in a couple of ways.  It can result in an irregular
sprue cut and “fining” around the perimeter of the base.   And
regardless of your casting technique, there are some moulds that
eventually collect a bit of lead between the mating surfaces.  Hence
the mould does not fully close, typically resulting in some whiskers or
fining down the sides of the bullet where the mould halves close and a
slight increase in the bullet weight.  In either of these situations, take
a break (ending the session) and clean the mould and/or adjust the
sprue plate.

Hopefully this article has provided some insight into why the weight
dispersion of your bullets may not be as small as you’d prefer.  There
are many casting “tricks” and techniques that were not discussed
such as how to better prepare you alloy, mould and ladle for easier
casting, but I believe most of the important ones that affect bullet
weights were covered.

And finally a word of advice for the newer BPCR shooters reading this,
don’t get discouraged if your batch of bullets have a weight spread of
3 to 4grs or possible more.  As you’re casting consider each step in
your process and how it may affect the outcome.  “Hang in there”
and continue to load and shoot those less than perfect bullets as
much as you can.  Range time (aka trigger time), especially attending
matches, will do a lot to refine your shooting technique and overcome
most bullet weight variations.  I believe I’ve developed a well refined
casting process but have yet to shoot a master score although I’ve
been close a few times.  I expect to make master class in big bore
silhouette competition one of these days but until then I’ll continue to
blame my spotter.

Wishing you perfectly cast bullets and great shooting,