TexasMac's Web Site
By Wayne McLerran
Updated 2/6/18

Over recent years I’ve completed a good bit of testing of rimmed
straight-wall black powder cartridges that headspace (see note below)
on the rim.  The testing consisted of: firing primed only cases, primed
cases with different non-explosive fillers and a bullet, black powder
cartridges with over-primer wads (OPW), BP cartridges with various
thicknesses of under-primer wads (UPW) and thousands of cartridges
with various powder charges and a variety of bullets.  The result is a
pretty clear understanding of the progression of events when a
cartridge is fired.
- All the testing and shooting and the following observations are based
on the use of standard large pistol primers.  Using magnum pistol or
large rifle primers may have different results.
- Headspace: SAAMI defines it 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”
- Head clearance: SAAMI defines it as, “The distance between the head
of the fully seated cartridge or shell and the face of the breech bolt
(breechblock) when the action is in the closed position”.
- For a discussion on using OPWs and UPWs, see the article at the
following link:

There are several conditions that affect how a cartridge reacts when
fired including: chamber/throat dimensions, chamber conditions, how a
cartridge is loaded and the components used.  Generally, assuming the
bullet nose is not pressed into the leade, when the firing pin strikes the
fully seated primer the cartridge is shoved forward until the rim makes
firm contact with the rim seat.  As the primer ignites, the force from
the primer alone is more than sufficient to drive
it backward out of the
primer pocket until it contacts the breechblock resulting in primer
setsback the amount of the head clearance.  The primer explosion
simultaneously forces the rear of the powder column forward and
ignites it.  As the powder ignites and chamber pressure increases, the
case expands and the bullet obturates (expands in diameter and
shortens slightly), “locking” the case against the chamber walls prior to
moving forward.  Under full pressure the cases stretches back some,
contacts the breechblock and reseats the primer flush with the rear of
the case head.  If the cartridge overall length (COAL) is such that the
bullet nose is pressed into the leade than head clearance is eliminated
and the primer has no room to setback.

If an UPW is used to cover the flash hole, the primer explosion forces
the wad forward.  The primer flame jets around the wad, igniting the
powder.  Relatively thin (0.004” or less) paper UPWs will usually be
burned up by the primer flame and/or burning powder.  Thicker UPWs
will be scorched but remain intact and either remain in the case, be
sucked into the bore or completely out the barrel by the pressure
differential as the bullet moves forward.  A primer will generally not
“punch through” a UPW thicker than 0.004” or so.  Remember, these
observations are based on using relatively mild large pistol primers, but
I would expect similar results from "hotter" primers.  If an OPW is used,
the wad is seated and supported by the bottom of the primer pocket.  
Due to primer pocket limitations the OPW is typically no more than
0.008” thick when used with large pistol primers in rifle cartridges.  
Since the OPW cannot move forward the primer explosion will pierce
the wad igniting the powder.

Next, the case expands as the powder column ignites and chamber
pressure rapidly increases.  Assuming the case was full-length resized, it
will expand several thousands and, depending on the case size; expect it
to shorten 0.005” to 0.010”.  If the case was fire formed and not
resized, typical expansion will be around 0.001” and shortening will be
negligible.  As the case expands and the bullet obturates and starts to
move forward along with the unburned powder and over powder wad,
the case will grab the chamber wall assuming the chamber is not
excessively wet or oily.  Although the case locks against the chamber
wall the pressure is sufficient to stretch it back enough to reseat the
primer flush with the rear of the case.  By the way, if you doubt the
case is stretching back to reseat the primer, fire an unloaded case with
only a primer.  If the primer remains slightly backed out of the primer
pocket but is flush after being fired in a full powder load, it’s a clear
indication the case is stretching back slightly, but it’s minimal and is not
a reason for concern assuming the headspace and/or head clearance is
not excessive.

If the chamber is coated with oil or excess moisture from cleaning,
wiping or from using an improperly designed blow tube, the now
forward-moving bullet may pull the neck portion of the case forward
while the rest of the case slides back, potentially resulting in a
stretched, split or completely separated case.  Other factors that
increase the possibility of stretching, especially with dirty or rough
inside case walls, are: relatively long cases, heavy powder column
compression which will increase powder-to-case-wall friction, over-
powder wad-to-case-wall friction, using LDPE or HDPE (poly) over-
powder wads, bullet-to-case-neck friction resulting from heavy neck
tension, bullet crimping and necks that are too soft due to excessive
annealing.  For an in-depth discussion on case stretching see the article
at the following link:

If the chamber pressure is insufficient to fully obturate the bullet and
tightly force the case neck into the chamber wall, a dirty outside case
neck will result due to burnt powder residue being blown back between
the case and chamber.  The most common method to correct this
condition is to anneal (soften) the case necks, which reduces the
amount of pressure necessary to expand the neck against the chamber
wall.  See the article titled,
Annealing BPCR Case Necks.  Another
method is to raise the chamber pressure by increasing the powder
charge.  Other techniques that may work include delaying bullet
movement using one or a combination of the following: seat the bullet
so that the nose is “hard into the leade” when chambered, crimp the
bullet, and/or increase the case-to-bullet neck tension.  But be mindful
that, as discussed earlier, excessive crimping and neck tension can
result in case stretching.

Once the bullet moves forward, the chamber pressure drops and the
case retracts (pulls away) from the chamber wall slightly (diameter
decreases by 0.001” to 0.002” at most), allowing for easy extraction.

That’s my story and I’m sticking to it.

Wishing you great shooting,