TexasMac's Web Site
CASE WALL UNIFORMITY MATTERS IN A
"TIGHT CHAMBER"
By Wayne McLerran
Posted: 12/12/20

Many of you have read my black powder cartridge rifle (BPCR)
articles, mostly relating to the Browning 1885 BPCR rifles.  But I also
own a very nice Shiloh Sharps 1874 BPCR in caliber .40-65 (Image 1).  
I won a certificate for the rifle at the 2018 Texas State match and
used the rifle in the same match in 2019 to win the scope category.  
There’s no question that it’s a very high-quality firearm and I enjoy
shooting it at matches along with my Browning’s.  It’s highly accurate
with the right loads as can be said for most custom rifles.
After shooting the Sharps in several matches I’d been experiencing
progressively harder chambering with some cartridges.  The rifle has
what I consider a “tight chamber”, meaning that a bullet of 0.001 to
0.002" over groove diameter just barely slides into a fireformed
unmodified case neck.  To be more specific, the chamber mouth
diameter of the Sharps .40-65 is 0.4325.  Since the case shrinks about
0.001" in diameter after firing and the Remington case walls at the
mouth are 0.011" thick (0.022" inclusive), the result is a case mouth
outside diameter (OD) of 0.4315.  Subtracting 0.022" from 0.4315"
results in 0.4095".  Therefore a 0.4090" to 0.4095" diameter bullet can
be thumb seated without resizing or enlarging the case mouth.  A
properly dimensioned tight chamber is good, accuracy wise, assuming
the brass is uniform.  By the way, I'm using the term mouth because
the chamber in my rifle does not have a well-defined cylindrical neck.  
Starting in front of the rim seat the chamber diameter drops off
(tapers) uniformly from 0.500” to 0.436” at about ½” from the
chamber-to-throat transition step, at which point the diameter
reduction continues but at a reduced rate, diminishing another
0.0035” over the last ½”.  Incidentally, the Remington cases were
reformed from .45-70 brass.

As to what was causing the hard chambering, I had no idea.  Prior to
loading the first case ever shot in the rifle the maximum case overall
length (COAL) was determined, so I knew it was not the cause and
neither was the bullet or seating depth.  The same COAL and bullet
has been used prior to the hard chambering issue.  So my next thought
was could it be the chamber dimensions?  I didn’t think so because if it
was than the hard chambering would have been an issue from day
one, but it was easy to find out.  Fortunately I had made a very nice
CerroSafe chamber cast (Image 2) shortly after receiving the rifle.  A
practice I always follow with a firearm I plan on keeping.  For my
article on making chamber casts go to:
http://www.texas-mac.
com/Chamber_Casts_and_Impact_Impression.html.
Using a RCBS CaseMaster Gauging Tool, the chamber cast was
checked using a technique similar to the process reloader’s use to
gauge bullet and/or case concentricity a.k.a. run-out (Image 3).  The
cast was carefully measured from the rim seat to slightly less than 1”
into the bore.  The result is an example of what I consider a perfect
chamber in perfect alignment with the bore.  It is flawlessly round
and the runout from the rim seat to 1” into the bore was no more
than 0.0005”.  It’s the kind of chamber one would expect to see in a
high-quality custom rifle.  So the chamber was definitely not the
problem.  The next thing to check was the fireformed cases.
Several of the fireformed cases were checked for roundness using the
RCBS gauge (Image 4).  The case wall thickness was then measured
with a tube micrometer (Image 5).  All of the easy-to-chamber cases
displayed a uniform roundness and uniform case wall thickness, not so
with the hard-to-chamber cases which were out-of-round from
0.001” to 0.002”, depending on the case.  And the wall thickness
varied approximately 0.001” (0.011” to 0.012” thick) when measured
with the tube micrometer.  DYKEM machinist layout fluid was applied
around the forward section of several cases to determine where they
were contacting the chamber wall.  By the way, A Sharpie brand or
permanent marker pin should work as well.
So how can the outside diameter of the case become out-of-round
when fired in a perfect chamber?  When fired the case diameter will
expand to fully fill the chamber then contract (shrink) slightly
(typically 0.001”) once the pressure drops.  Due to the difference in
case wall thickness the shrinkage is not uniform around the case,
resulting in the raised area on one side.  When a reloaded cartridge
with the now out-of-round fireformed case is pressed into the
chamber it contacts the chamber wall at two points as illustrated
(Image 6).  The high point on one side of the case pushes the mouth
away from the chamber, forcing hard contact of the case mouth or
lip on the opposite side.  The bullet prevents the case neck and
mouth from flexing sufficiently to allow easy entry of the cartridge.
Techniques to resolve the problem:

Following are some suggestion on how to temporarily or permanently
eliminate the problem:

Trash the reformed Remington cases and purchase Starline.  In an
earlier exercise I dissected Remington, Winchester and Starline brass
and found that Starline case wall thickness varied less than the others.  
To read the article, go to:
http://www.texas-mac.
com/Comparison_of_Remington_Winchester_and_Starline_Brass.html

Use Tim Smith-Lyon’s case stretching tool to swage (squeeze) and
uniform the case necks, then trim the length if necessary.  Be aware
that, in addition to uniforming the case wall, the process will thin the
walls slightly; resulting is possible lengthening of the case on the side
with the thicker wall.  To read an article on case stretching using the
tool, go to:
http://www.texas-mac.
com/Lengthening_Short_Brass_To_Fit_Your_BPCR_Chamber.html.

Use one of the two processes discussed in the article titled,
Removing Case Peened Edge (
http://www.texas-mac.
com/Removing_Case_Peened_LIp_Edge.html).  It will not fully remove
the thicker wall but should reduce the thick area slightly to ease
chambering.  It may be necessary to repeat the process after each
firing.

After neck resizing and expanding, use an outside neck turning tool
(Image 7) to uniform the neck wall thickness.  The resizing and
expanding steps should create a neck with a constant inside diameter
which is necessary when using the outside neck turning tool.  The
problem should be completely eliminated after removing only the
minimum material necessary and subsequently fireforming the case.
So there you have it.  An example of the type of situation that one
can encounter when loading custom ammo for a custom chamber, a
chamber designed to maximize the accuracy potential of a custom
target rifle.

Wishing you great shooting,
Wayne