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Last update: 8/26/14

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 section.  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, the preferred chamber cast
material which will be discussed later, using a melted mixture containing mostly
sulfur, commonly known as the “Baker Mixture”, was the time-honored technique.  
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.  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.  
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:
http://www.youtube.com/watch?v=j8jE2o7ePbo.

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 containing

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:
http://www.youtube.com/watch?
v=2d5DA4_OcuE.  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 vernier 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 pouring tube and
funnel.  The tube and funnel can be made from brass, copper, aluminum or steel
tubing and should be as short as possible to minimize CerroSafe solidifying in the
tube.  Use an existing metal funnel or make a temporary one out of thin sheet
metal, brass shim stock or a few layers of aluminum foil (the easiest method).
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 damage.
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 pouring.
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 enlarges.
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.
Some 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 formula 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 formula is X = 0.461” ÷ (1 +
0.0051).



















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 benefit applies to both the sulfur cast and the 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 take in account headspace.  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 have 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”.  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,
Wayne
CHAMBER CASTS & IMPACT IMPRESSIONS
By Wayne McLerran