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So if you’ve been holding on to old CerroSafe chamber casts, pull
them out, take some careful measurements and apply the formula and
expansion factors listed above.  The results should approximate the
chamber, throat and bore dimension of your firearms.

Advantages & Disadvantages of CerroSafe Chamber Casts
So, what are the advantages and disadvantages to using CerroSafe?  
One advantage is the relative hardness of the cast.  It’s not as easily
damaged as an impact impression which is discussed below.  If the
cast is sufficiently long to include a short section of the lands, it will
provide dimensional data of the throat and bore, but the same
information is also available from a sulfur cast and impact

impression.  Also, a CerroSafe cast provides diameter data along the
entire chamber, including the center and rear portion of the chamber;
data not available from an impact impression.  For most shooters this
data is not particularly important so I don’t consider this much of an
advantage, if any.

Disadvantages include: handling concerns and hazards; dealing with
expansion characteristics; having to partially or fully disassemble the
action to pour and remove the cast; the inability to directly measure
maximum case length since the technique does not provide headspace
dimensions.  The latter two are the main reasons I prefer an impact
impression.  For a discussion on measuring headspace, refer to the
chapters titled “Chamber Terminology” and “Browning’s Chambers”.

Impact Impressions

In lieu of using sulfur or CerroSafe for a chamber cast, making an
impact impression is an excellent method, which is also commonly
referred to as making an impact casting or impact slug.

Making an Impact Impression
1.  Starting with a fire-formed case, shorten the length by
approximately 0.200” (the amount is not critical) and fill it with lead
to approximately ½” below the mouth.
2.  You’ll need an additional amount of pure or nearly pure lead
slightly smaller than the bore diameter.  It will be dropped down the
bore and compressed to form the impression.  If bullet lead is not
available, soft lead wire can be found in the fishing section of most
sporting goods stores.
3.  Clean, lightly oil the chamber and insert the partially lead-filled
fire-formed case.
4.  Next you’ll need a steel rod just under bore (land) diameter,
available at most hardware stores.  The rod should be a few inches
longer than the barrel with a squared-off cleanly finished end.  Wrap
the rod just above the end with tape to make a snug fit with the
bore, helping to center the end of the rod and eliminate any
possibility of the edge galling against the bore.  If a rod of the proper
diameter is used, the possibility of damaging the rifling lands is just
about nonexistent.  Taping the end is just an extra precaution in case
the edge of the squared off end is irregular.  The only other location
I'd consider taping is the section that comes in contact with the
muzzle to protect the crown.
5.  With the rifle standing up and butt stock on the floor, drop a
sufficient quantity of lead down the muzzle to fill the case neck,
chamber and throat when the impression is formed.
 Be conservative
and don’t use too much lead or the impression will be too long,
extending down the bore and hard or impossible to remove without
damaging it.  If after removing the impression the amount of lead was
not sufficient, it’s easy to redo with the addition of more lead.
6.  Using a piece of paper towel, insert a wad of around 1/2" into the
muzzle end.  The paper will compress, fill the grooves and prevent the
rod from embedding itself and seizing on the end of the compressed
lead impression.
7.  Insert the end of the rod into the muzzle until it contacts the
paper wad.  Now strike the rod with measured even stokes, just
enough to fully compress the soft lead.  When the lead is fully
compressed a difference will be felt as the rod will start bouncing
with every stroke of the hammer.
8.  Open the action and gently push the case and lead impression out
with the steel rod.  Fold up a soft cotton rag or towel to catch the
impression as it’s pushed out of the chamber.  Be gentle so as not to
further deform the lead impression.  If the correct amount of lead
was used, the impression should neatly form to the dimensions of the
neck and mouth of the chamber, the throat and a small section
of the bore.  If it’s not long enough, repeat the process after adding
more lead.
Note: When adding additional lead to an existing impression, I've had
good success with roughing up the end of the lead impression,
reinserting it, adding additional lead down the bore and swaging a
new impression. In all but a couple
of cases the new lead firmly
adhered to the old impression and the seam was virtually invisible.
The couple of times that the added lead did not stick well were most
likely due to oil contamination.  If, after roughing up the end of the
old impression, there’s still a problem with the added lead adhering,
start over
by cutting off the lead extending out of the case and drill a
little out of the mouth to a depth of .100” to .200”.  Assuming the
primer hole is plugged, an easier method may be to heat up the lead-
filled case and pour a little out.

Advantages & Disadvantages of Impact Impressions
There are several advantages using this technique. It’s cheaper,
simpler and easier than using CerroSafe.  The rifle action does not
require disassembly and there’s
no chance of spilling excess material
into portions of the action.  Measuring the impact impression from the
rear of the rim to the start of the chamber transition includes the
actual headspace of the rifle and will directly provide the correct
case over-all length (OAL) of a properly trimmed fire-formed case.  
Plus, if the impression is sufficiently long to include a short section of
the lands, it will provide dimensional data of the throat and bore.  
Finally, there’s no concern about the impression changing dimensions
over time.

There are a couple of disadvantages which I feel are not significant,
but should be discussed.  Being made from soft lead, one negative is
the ease of damaging the impression if it’s not handled with
reasonable care.  Of course, if damaged it’s relatively simple to make
another one.  Another disadvantage is the inability to accurately
measure diameters along the center portion and rear of the chamber;
data that’s generally not required or useful for most shooters.

Wishing you great shooting,
By Wayne McLerran
Last update: 3/10/20

Table of Contents
Sulfur Chamber Casts
CerroSafe Chamber Casts
Comments & Cautions
Making a Fusible Alloy Chamber Cast
Adjusting Data Taken From “Aged” Casts
Correctly Adjusting Data taken from “Aged” Casts
Advantages & Disadvantages of CerroSafe Chamber Casts
Impact Impressions
Making an Impact Impression
Advantages & Disadvantages of Impact Impressions

Chamber casts and impact impressions are invaluable solutions for
measuring the chamber and throats of the vast majority of rifles.  
They are also ideal for determining accurate dimensions for bullet
moulds, bullets and case lengths.  The steps involved in making a cast
or impression are detailed in this article.  The advantages and
disadvantages of each technique are discussed to help the reader
decide the best method to use.

Sulfur Chamber Casts

Prior to the development and use of CerroSafe for chamber casts, the
time-honored technique used a melted mixture containing mostly
sulfur, commonly known as the “Baker Mixture”.  Sulfur does offer a
couple of benefits: it’s cheap and readily available from local home
and garden supply stores or pharmacies.  Although high purity
pharmaceutical grade sulfur is not required for a chamber cast, it is
available in several forms.  Flowers of sulfur, also known as sublimed
sulfur is made by sublimation and is the most common.  Two other
forms are precipitated sulfur, also known as milk of sulfur, and
washed sulfur.

I have not used sulfur for chamber casts but researching the subject
highlighted several negatives.  It’s somewhat dangerous to melt and
can ignite with an almost invisible flame.  It can be very messy and
hard to clean out if the chamber is overfilled and the sulfur runs into
or is accidentally spilled into the action.  And sulfur casts are very
brittle and somewhat fragile.  In addition, when using sulfur casts to
determine case lengths, headspace cannot be directly measured.

Roy F. Dunlap covers the technique of using sulfur in his very well-
know book titled Gunsmithing, first published in 1950 and followed by
a 2nd edition in 1963.  Videos detailing the process are also available
on YouTube.  Following
is one shooter’s procedure I found during an
Internet search.

The chamber and a portion of the bore should be clean and swabbed
with a very light coat of lubricant or WD40.  From the muzzle end,
push a tight patch down
the bore to about ½” ahead of the throat.  
After removing the stock and/or
forearm, mount the well-padded
receiver and barrel in a vise.  Slowly melt the sulfur over low heat,
being cautious that it does not catch on fire.  Heat the barrel and the
chamber area with a propane torch or hot air gun until it’s just too
hot to hold.  Plug the extractor slot.  Now carefully pour the melted
sulfur into the chamber all the way to the top. A funnel shaped
depression will appear in the center which is normal. Wait several
minutes for the chamber to cool some and gently knock the cast out
with a rod from the muzzle end.  Be sure to catch it.  If measurements
are taken immediately, the cast will be about .0001” smaller than
actual chamber and bore dimensions.  Within a few hours the cast will
not shrink more than .0005” which is certainly close enough.  After
more time has passed
the cast will eventually shrink .001”.  Here’s a
link to a YouTube video on making a sulfur chamber cast:

CerroSafe Chamber Casts

Comments & Cautions
The most common chamber cast materials in use today are “fusible
alloys”, generally referring to a metal alloy that melts at relatively
low temperatures; typically below 361°F (the melting point of lead tin
eutectic solder).  The best known fusible alloy for firearm chamber
casts is sold under the CerroSafe brand.

Prior to discussing the steps involved in making a CerroSafe chamber
cast, I will briefly cover some background information on the alloy and
the company that currently produces it.  Until 2007, CerroSafe was
made by Cerro Alloys, hence the CerroSafe brand name.  Bolton Metal
Products acquired Cerro Alloys from Cerro Metal Products in February
2007.  The fusible alloy is now officially listed on their website as
Bolton 160/190 in reference to the melting range but it’s still supplied
in ½ lb ingots under the CerroSafe brand.  Bolton Technology is
headquartered in Bellefonte, Pennsylvania.  Although RotoMetals,
another manufacturer, calls their version Chamber Alloy (158-190),
throughout the following discussion I will refer to the alloy as
CerroSafe for simplicity.

CerroSafe is comprised of bismuth (42.5%), lead (37.7%), tin (11.3%)
and cadmium (8.5%).  Bismuth is the essential ingredient, the
percentage of which determines if the alloy will shrink or expand after
a cast is made.  CerroSafe has a melting range of approximately 158°F
to 190°F.  It’s available from gunsmith supply companies and several
firearms accessory retailers in ½ lb or larger ingots.  I’m currently
aware of five retailers.  Brownells, MidwayUSA, Buffalo Arms and
Track of the Wolf sell CerroSafe supplied by Bolton Metal Products.  
RotoMetals manufactures their version of a fusible alloy, reportedly
using the same alloy formula used in CerroSafe.  Per equivalent
weight, RotoMetals current price is about 1/2 that of CerroSafe.  
There may be other suppliers.  The typical fusible alloy cast of a .45-
70 chamber, including the throat, weighs approximately 3.4 oz; a .45-
90 cast a little more, a .40-65 cast a little less.  Therefore a ½ lb
ingot should be more than sufficient to make two casts.
A few words of caution are warranted concerning handling an alloy
lead and cadmium.  We are all aware of the harmful result
of inhaling or digesting lead.  Cadmium is even worse.  Although the
percentage of cadmium in CerroSafe is relatively small, it’s
considered toxic even in low concentrations and will bioaccumulate in
organisms and ecosystems.  As an example, the permissible exposure
limits (PEL) of lead is 0.05 parts per million (ppm).  For cadmium it’s
only 0.01ppm.  Repeated or prolonged exposure to cadmium can
damage the kidneys, lungs and/or liver.  So use extra caution and be
sure to wash your hands when melting and handling CerroSafe or
similar fusible alloys, or casts made from fusible alloys.

If all you need is one or two casts and have no plans to measure the
cast dimensions again, ignore the discussion on CerroSafe’s expansion
rates except for the important 1.5 hr measurement point.  But if you
plan on keeping the cast for future measurements than a clear
understanding of the expansion characteristics are necessary to
ensure the data accurately represents the true dimensions of the
chamber, throat and rifle bore.  An in-depth discussion on expansion
characteristics follows the details on making a cast.

Making a Fusible Alloy Chamber Cast
The following steps detail how I make a chamber cast.  No doubt
there are many variations used by others.  For one example, here’s a
YouTube video made by Midway USA’s Larry Potterfield:
v=KgRp3r9VPE0.  Larry leaves out some of the
details but the video will provide a general idea of the process.

It’s well known that once the cast is poured CerroSafe shrinks and
then starts to expand over time as it “ages”.  Some suppliers note
that the cast will match the chamber dimensions 1hr after the cast is
made.  In a controlled experiment I determined 1.5 hrs is a better
estimate of the ideal time to take measurements.  Measurements
taken before or significantly after 1.5 hrs will require a mathematical
adjustment.  There’s no need to rush, a few minutes one way or the
other won’t make a significant difference, but 30 minutes to an hour
will if precise measurements are required.  Have a caliper or
micrometer handy and determine what data is needed prior to the
measurement time.  If the preferred time interval is missed,
adjustments to the data will be necessary (see later instructions) or
remake the cast.

1.  Disassemble the rifle as needed to gain access to the chamber.
On many single-shot rifles it is only necessary to remove the forearm
in order to heat the barrel chamber and throat.  Although
disassembling the firearm is not mandatory and may not be
recommended for some rifles, I find it simplifies and helps when
making a chamber cast for two main reasons: it allows sufficient
access to pour the alloy in the chamber without the need of a pouring
tube and also eliminates or at least reduces the possibility of spillage
into parts of the action.
 A rifle that does not offer good chamber
access may require a funnel
and tube to reach the chamber.  If made
out of metal it should be as short as possible to minimize the alloy
solidifying in the tube.  Another option is to
make a funnel out of a
few layers of aluminum foil (the easiest method).  Better yet, make a
pour tube or funnel out of heavy paper or thin cardboard.  The fusible
alloy should not be hot enough to burn the paper which will not
conduct heat away from the alloy.
2.  Clean and lightly oil the chamber, throat and at least a short
section of the
bore forward of the throat.
3.  Fold up a cotton cleaning patch or section of paper towel and roll
or form it into a cylinder shape until it will tightly fit into the bore
and act as a plug/dam for the melted alloy.  I prefer to insert the
patch from the muzzle, but it can also be inserted from the breech or
chamber end of the barrel.  Using a cleaning rod, force the patch
down the bore until it reaches a point approximately 3/4” to 1” in
front of the mouth or throat of the chamber.  Ideally you’d like to see
an inch or so of the rifling when the cast is made.
4.  Secure the rifle vertically (chamber end up) in a padded vice or
similar holder while taking precautions to protect the rifle from
5.  Heat the barrel around the chamber with a propane torch or hot
air gun to a point that it’s good and warm but still comfortable to
hold by hand.  I’d say 125 degrees is about right.  Do not overheat or
get it so hot that it’s uncomfortable to hold by hand.
6.  Melt the Cerrosafe.
 There are many methods to melt CerroSafe.  
Use a double boiler or use other sources of indirect heat.  Caution -
applying direct heat and/or overheating may lead to separation of the
components of the alloy or lead to the release of dangerous fumes.  A
large clean bullet casting ladle or plumbers ladle can be
used to heat and pour the alloy, using a propane torch to heat the
ladle from the bottom.  I prefer to place the CerroSafe in a small
“tuna fish” can and use a hot
air gun for heating.  The can is bent to
form a convenient pouring spout and
placed on a heat resistant
surface such as a brick.
 “Vice-Grip”-type locking pliers can be used
as a handle for the can.  Since the melted CerroSafe should not be
much more than 190 degrees F (less than the temperature of boiling
water), I prefer to use heavy leather gloves when
handling the tuna
can.  It provides for better control while pouring.
7.  Don’t forget to plug the extractor slot or a real mess will result
when the alloy cascades down the slot into the action or receiver.
Tip: A common “Q-Tip” or cotton tip applicator works great if the
slot is not too wide.
8.  Slowly pour the alloy, completely filling the chamber until the
alloy covers the rim seat and forms a slight mound at the breech.
I’ve found that pouring too fast may result in very large voids in the
cast.  Over pouring will result in excess alloy spilling over the breech
and likely prevent removal of the cast, especially if the action has not
been disassembled.  When using a pouring tube it may be necessary to
have a helper heat the tube with a propane torch or hot air gun while
9.  Wait at least 10 minutes for the cast to solidify and shrink to push
it out of the chamber.
 The maximum shrinkage time is 20 to 30
minutes after which it will start to enlarge.  Waiting any longer and
the chances increase that the cast will not come out as it further
10.  Flip the rifle over (muzzle pointing up) and secure it in the vice.
11.  While waiting to remove the cast fold up a soft rag or towel and
place it on the floor below the inverted rifle.
 The towel will gently
catch the cast as it’s pushed out of the chamber.
12.  Remove the cast using a cleaning rod inserted from the muzzle
and tapped with the heel of your hand.
 If the cast will not budge and
something heavier would be required to drive the cast out than you’
ve probably waited too long.  In that case, reposition the rifle with
the breech up and heat the barrel with a propane torch or hot air gun
sufficiently to melt the alloy.  Allow it to cool for ½ hr and try
tapping it out again.  If it will not budge on the 2nd attempt you’ll
have to heat the barrel sufficiently to melt the alloy and pour it out
of the barrel.
13.  With the cast in hand, check out the surface condition.
small voids and frosting is normal, but many wrinkles or large surface
voids are an indication that the alloy was not hot enough, the barrel
was not sufficiently warm or the alloy was poured too fast into the
chamber.  Heavy frosting on the surface is an indication of
overheating of the alloy or barrel.  If either of these conditions
prevents good measurements, make a new cast.

So now you’ve successfully made a CerroSafe cast.  Having taking
careful measurements after waiting the recommended 1.5 hrs, you’ve
decided to keep the cast for future measurements.  In that case, be
sure to document the date and time the cast was made.  I apply
Scotch tape around the middle of the cast and write the info on it
with a permanent ink felt tip pin.  I’ve inadvertently mixed
up casts
of the same caliber and find this to be a better method than labeling
a storage envelope.  Better yet, do both.

Adjusting Data Taken From “Aged” Casts
Most shooters make a chamber cast, take some measurements and
never use the casts again for various reasons.  The most obvious
reason is, once accurate measurements are made and documented at
the correct time, there’s little or no need to revisit the cast.  Another
reason is they are aware that the cast will expand over time but are
unsure how to determine the original dimensions from the aged and
expanded cast.  For several reasons I’ve had the need to use old
CerroSafe casts to verify original measurements and/or take
dimensional data from different locations than originally measured.  
In doing so I came to the conclusion that the published expansion data
are flawed as is the method to use it.  The bottom line is the
expansion data that has been published to date is very old (dating
back to 1934) and I experimentally determined it to be incorrect.

Correctly Adjusting Data taken from “Aged” Casts
Based on the experimental data, the expansion factors listed in the
table below are what I now use.  The factors are in inches per inch.  
Therefore, to calculate original dimension (X) from aged dimension (Y)
use the equation X = Y ÷ (1 + Factor).  For a close approximation use
X = Y (1 - Factor).  For example, using 0.461” for the groove diameter
of a 200 hour or older age 45 caliber cast will result in a value of
0.4587”.  For this example the equation is X = 0.461” ÷ (1 + 0.0051).