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By Wayne McLerran
The shooter indicated he hastily cleaned the chamber and bore prior
to the shots and may have not cleaned all the moisture out.  Plus he
was using a blow tube without the “O” ring seal that it was designed
for.  But he contributes most of the reason for the separation to using
a case with excessive neck tension due to lip peening when the cases
were cleaned with stainless steel pins.  Refer to the article titled
STAINLESS STEEL PINS.  He said “I normally do not use neck tension as
I slip fit my bullets (custom made for my bores) into fire-formed
cases.  The cases that separated were fire-formed, but because of the
SS media the neck or should I say the mouth was extremely tight to
the point I had to use my press to seat the bullets also shaving some
lead at the mouth of the cases.  I should have stopped at that point &
tried to evaluate what was going on but it had been some time since I
loaded for the .40-65 so I brushed it off.”

I’m betting it was a combination of excessive chamber moisture and
excessive neck tension that led to the separations.  The shooter plans
on using ceramic media to clean cases and a much better fitting blow
tube from now on, and no doubt will be paying much closer attention
to his reloading process.  It has also been suggested that a
contributing factor may have been work hardening of the case from
many firings, therefore the case necks may need annealing.  For more
details on annealing, see the article titled

There are several factors that determine if a cartridge case will
stretch and/or completely separate when fired in a BPCR.  But before
jumping into a discussion on case stretching, a few terms need to be
defined.  One is headspace, defined by the Sporting Arms and
Ammunition Manufacturers Institute (SAAMI) as
“The distance from the
face of the closed breech of a firearm to the surface in the chamber
on which the cartridge case rests”
.  I prefer the definition found in
Speer’s reloading manual,
“The distance from the surface of the
barrel or chamber that positions the cartridge and prevents its further
forward movement into the chamber, to the face of the bolt or
.  Therefore, in full agreement with both definitions, the
headspace in a rimmed cartridge rifle is measured from the face of
the bolt or breechblock to the bottom of the “rim seat” which stops
the cartridge from moving forward.

Another term is head clearance which SAAMI describes as,
distance between the head of a fully seated cartridge or shell and the
face of the breech bolt when the action is in the closed position…”.
Although distinctly different as defined above, headspace and head
clearance are commonly confused with each other.  Firearm
manufactures try to keep the head clearance as small as possible, but
some clearance is needed to allow for variances is case rim thickness.

Depending on how a cartridge is loaded, there are many variables that
affect how it reacts under firing conditions.  Generally, when a
rimmed cartridge is fired the firing pin strikes the fully seated primer
and shoves the cartridge forward until the rim makes firm contact
with the rim seat.  When the primer ignites, the force from the primer
alone is more than sufficient to drive it back out of the pocket until it
contacts the breechblock.  As the powder ignites and pressure builds
up, the case will expand and should “lock” against the chamber walls
assuming the chamber is not wet with water or lubricant, resulting in
permanent primer setback unless the case stretches back and reseats
the primer flush with the rear of the case head.  If case stretch is
negligible, than the amount of primer “setback” will be equal to the
head clearance.  If the case does stretch back and flush seats the
primer than the amount of case stretching is also equal to the head
clearance.  If you’re wondering if cases are stretching enough to
reseat the primer, run the following test.  Fire an unloaded case with
only a primer.  If the primer is backing slightly out of the case but is
flush after firing normal full power loads, than it’s an indication that
the cases are stretching slightly or possibly sliding back.  If they are
stretching they may return to the original length, or even shorten
some after firing, especially if they were full-length resized.

There are several factors that contribute to case stretching, some
much more than others.  Two important ones are chamber pressure
and case shape, but there are others including but not limited to:
head clearance amount, case length versus chamber length, over
powder wad-to-case-wall friction, bullet-to-case-neck friction
resulting from neck tension, dirty or rough inside case walls, powder
column friction, bullet crimping and necks that are too soft due to
excessive annealing.  In addition, anything that prevents the case from
sufficiently expanding and gripping the chamber wall can result in case
stretching.  Some examples are: brass tensile strength, hard brass or
brass that is work hardening due to multiple firings and resizing,
chamber wall lubrication (see note below), and even breech seating
(see note below).
- If the chamber wall is over lubricated from oils or with excess
moisture from an improperly designed blow tube, cases can stretch
both backward and forward; more on this later.  Wiping the bore
between shots and not using a “drying mop” or other techniques to
remove moisture from the chamber can result in separated cases.  I
wipe between shots and, to save time, have modified the wiping rod
to include a slot for the drying patch to fully enter and dry the
chamber as the wiping patch is pushed out of the bore.  The drying
patch is replaced between relays.
- Breech seating is a loading technique whereby the bullet is
separately loaded into the chamber/throat followed by loading a case
full of powder.  Since the bullet is not in the case neck, when fired
there may not be sufficient back pressure to expand the case to
tightly grip the chamber wall, resulting in the powder column and wad
friction trying to stretch the case forward.

I’ll start with case stretching in BPCR rifles chambered for
“bottleneck” ammo - ammo in which the cartridge case has a well
defined shoulder.  BPCR bottleneck cartridges typically use “rimmed
cases” which headspace on the front of the rim.  When fired, high
pressures will try to force the case body to stretch in two directions:
backward until the rim contacts the breechblock and forward until the
shoulder contacts and fully forms into the chamber shoulder.  
Depending on how well the case dimensions match the chamber
dimensions, the front portion of the case can be stretched forward by
the pressure from the forward moving unburned powder column
hitting the shoulder, which is the main reason BPCR bottleneck cases
tend to stretch more than BPCR straight wall cases.  Some stretching
can also take place in the neck due to “bullet pull” or friction
between the case, wad and/or bullet caused by neck tension.

Bottleneck cases can separate in the neck region but it’s more likely
that the case will separate in front of the thick web forward of the
rim, commonly referred to as case head separation, after being fired
and full-length resized several times.  Since the case stretches some
when fired, full-length resizing is generally used to “set back” the
shoulder to the correct length resulting in the overall case length
being slightly long, and trimming is required.  The stretching and
resizing process forces material from the case walls to migrate
forward.  Eventually the wall in front of the web is too thin and
separates as the head (rim and web) is driven back by chamber

So what about case stretching in straight wall ammo?  Case head
separation can occur but it’s less likely than in bottleneck cases.  
Cartridges with straight-wall rimmed cases also headspace on the
front of the case rim.  With no shoulder there's less forward pressure
on the case walls and less stretching forces on the case, again
assuming the chamber wall is not excessively lubricated and the inside
walls of the case was clean prior to loading.  Properly cleaned BPCR
straight wall cases may stretch slightly but generally it won’t be much
unless the cases are relatively long, bringing other factors mentioned
earlier into play.  For example, assuming similar loading techniques, a .
45-110 or .45-120 case is more likely to stretch than a .45-70 or .40-
65 case.  The stretching forces on the case will increase due to the
added friction between the longer case and longer powder column,
especially if the powder is tightly compressed.

Shorter cases can normally be full-length resized, reloaded and fired
many times before trimming is required.  But too much moisture or
lubricant on the chamber wall can cause a problem.  It’s somewhat
rare but as displayed in the photo above, if the chamber wall is highly
lubricated or has excess moisture, straight wall cases have been
known to completely separate with little or no prior visual warnings.  
Dirty or rough inside case walls can exacerbate the problem and LDPE
(poly) wads are known to expand in diameter under pressure and grip
the neck wall much tighter than paper or vegetable fiber wads.

Consider the following situation.  The inside of the case is corroded or
has not been properly cleaned of powder fouling.  A tight fitting poly
wad was used to separate the powder from the bullet.  Once the wad
was inserted a compression die was used to highly compress the
powder.  The cartridge case has a cannelure and the bullet is crimped
in place.  The rifle chamber wall has been cleaned and oiled and the
shooter forgot to remove most of the lube with a dry patch; or excess
moisture has accumulated due to the aggressive use of a poorly fitting
blow tube or the chamber was not dried after wiping.  As the
cartridge fires and chamber pressure increases, the forward portion of
the yet to be ignited irregular-shaped hard black powder granules are
forced against and tightly grip the dirty case wall.  Due to increased
compression the already tight poly wad tries to expand further and
tightly grips the dirty case wall.  The bullet fully expands before it
starts to move forward, forcing the case neck into full contact with
the chamber and increasing neck tension.  The bullet crimp adds to
the friction inside the case neck and mouth.  Under these conditions it
should not come as a surprise that the case stretches or the case neck
separates at the cannelure and is pulled into the bore by the bullet.

Cases with cannelures are more prone to separating due to the slight
thinning of the case wall when the cannelure was formed, but
cannelures are not required for case separation.  Although the above
is likely a worse-case scenario, separation can result if only some of
the noted conditions are present and the case may have to be fired
several times before it separates.

With straight wall cases, because no shoulder exists to serve as a stop,
if the case separates the front portion can be driven forward with the
bullet or pulled forward into the bore by the wad and/or bullet as
displayed in the photo above.  I've heard of situations where the front
section of the case completely exited the bore along with the bullet.  
It’s a common practice to purchase or make blow tubes for BPC rifles
that use a cartridge case at the end of the tube.  When doing so
ensure the case closely fits the rifle chamber to minimize excess
moisture accumulating in the chamber.  More on this and how to
construct a blow tube is discussed in the article titled
TUBE FOR BPCR SHOOTING.  Although I've never experienced a
completely separated straight wall cartridge case, I have had a couple
show signs of thinning in the middle.  At the time I was using a blow
tube with a full length resized case.

Since stretching is generally insignificant in properly cleaned and
loaded straight wall cases of the most popular cartridges, neck-only
case resizing does not provide quite the same level of benefits that it
does for bottleneck cases.  The main reason the technique is used with
straight wall cases is to obtain the correct case-to-bullet neck tension
while ensuring the brass uniformly matches the chamber dimensions as
close as possible for maximum bullet alignment.  Neck-only sizing also
minimizes resizing stresses on the rest of the brass, therefore reducing
work hardening of the brass and extending the case life.  For
additional information on resizing go to the article titled

Wishing you great shooting,
Updated 12/13/18

The photo below is an example of a split Winchester .40-65 case next
to an unfired case of the same original length.  Adjusting for the gap in
the photo, the case was still stretched another ¼” when the neck
section was pulled into the throat and part of the bore.  Clearly visible
is the chamber to throat transition step, the approximately .100” of
freebore, the leade and rifling impressions.  The shooter had two cases
separate from the same lot of reloads.  Both separated at the same
location.  By the way, the case had been reformed from original
Winchester .45-70 and trimmed to a length of 2.1”, about .025” less
than the chamber length, so the length was not a factor in the