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CASE STRETCHING & SEPARATING IN
BLACK POWDER CARTRIDGE RIFLES
By Wayne McLerran
Updated 8/2/14

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 separation.




















The shooter indicated he hastily cleaned the chamber & 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
CASE CLEANING - ULTRASONIC VS TUMBLER WITH CERAMIC MEDIA
& 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 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
ANNEALING BPCR CASE NECKS.

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 breechblock”
.  Therefore, in full agreement
with both definitions, the headspace of 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,
“The 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 “lock” against the chamber walls, 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.  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).
Notes:
- 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.
- 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.

So what about case stretching in straight wall ammo?  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 is 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.  
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
MAKING A BLOW 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
TO RESIZE OR NOT TO RESIZE.

That’s my “take” on the subject.


Wishing you great shooting,
Wayne