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FIRING A BLACK POWDER CARTRIDGE -
PROGRESSION OF EVENTS
By Wayne McLerran
Updated 5/20/15

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.
Notes:
- 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”
- For a discussion on using OPWs and UPWs, see the article at the following link:
http://www.texas-mac.com/Large_Pistol_Primers_and_Primer_Wads_in_BPCRs.
html

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, to name a few.  Generally, the firing pin strikes
the fully seated primer and shoves the cartridge 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 setback unless the case stretches back,
contacts the breechblock and reseats the primer flush with the rear of the case
head.  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.

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 no
reason for concern assuming the headspace 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:
http://www.texas-mac.
com/Case_Stretching_and_Separating_in_BPC_Rifles.html

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 raise the chamber pressure by
increasing the powder charge.  Another approach that may work is to delay 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,
Wayne