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SHOOTING "SHORT-CASE" STRAIGHT-WALL
BLACK POWDER CARTRIDGES - MYTHS & FACTS
CONCERNING CHAMBER RINGS & ACCURACY
By Wayne McLerran
                                             Figure 2

Another example; the Browning .40-65 BPCR has a chamber plus
head-clearance length of 2.125” – 2.130”.  New .40-65 brass is
typically 2.095” long and around 0.008” to 0.010” less in
diameter than the chamber.  When fired the case will expand to
match the chamber diameter and shorten around 0.008” to
0.010” assuming it was full-length resized.  The case will
immediately shrink about 0.001” in diameter, allowing for easy
extraction.  The fire-formed case is now larger in diameter but
shorter than when new, resulting in a mismatch or gap
approaching 0.040” in the Browning chamber.  Although much
smaller than the previous example, the gap is still considered too
large and may affect accuracy for the same reasons, but not to
the degree previously noted.  In this example the gap is relatively
narrow, less than the width of a typical lube groove.  If the bullet
is seated in the case so that an exposed lube groove aligns with
and covers the gap when loaded, lube will fill the gap rather than
lead, diminishing the possibility that lead may remain in the
chamber as a shallow ring or trapped and build up in the transition
step.  By the way, reloaders reforming .45-70 brass to .40-65
dimensions have an advantage.  After reforming, the case will be
longer than the .40-65 chamber and is easily trimmed to the
correct overall length to match the length of the chamber plus
head clearance.

Some new .45-70 brass is also slightly less than 2.10” long when
new and will shorten an additional 0.008 - 0.010” when fired (fire-
formed).  If the chamber dimensions are in compliance with SAAMI’
s specifications (some are not), the length will be very close to
2.10” and all is well.  When the cartridge is fired the small gap
created between the case mouth and the transition step is
relatively small and should not have a significant effect on
accuracy.  Although an exact match is the ideal solution, a well-
known shooter and experienced BPCR experimenter has
determined that it is good practice to ensure the gap between the
end of the fire-formed case and the end of the chamber does not
exceed 0.010”.

So, although “short-case” cartridges can be safely fired with
sufficient accuracy for hunting or even informal target practice,
for the above noted reasons it’s not a good idea if the goal is to
maximize the accuracy of the rifle.  To determine if your cases
are too short, see the article titled
DETERMINING CASE LENGTHS.  
So what’s the answer if you have a bunch of brass that’s too
short?  Depending on the cartridge you’re shooting there are
solutions, some simpler and less costly than others.  For a
discussion and evaluation of three techniques, see the article
titled
 LENGTHENING SHORT BRASS TO FIT YOUR BPCR CHAMBER.

Wishing you great shooting,
Wayne
Last update: 1/22/18

Having written and published a book on the Browning and Winchester
BPCR rifles I receive many questions and concerns from current owners
and those considering acquiring one of the rifles.  Although most of the
questions are specific to the Browning and Winchester BPCRs, many are
general in nature and come from both new and experienced shooters.  
What I found interesting and in some ways disconcerting is several of
the inquiries were due to myths concerning chamber rings and
accuracy that have been perpetuated as fact due to being passed
around from shooter to shooter and on Internet shooting forums.  I
thought I’d “take a shot” at debunking a couple of the myths and
confirm one regarding accuracy.  By the way, having no experience
with bottle-neck black powder (BP) cartridges and unable to locate any
data relevant to the following subjects, my comments pertain only to
straight-wall cartridges.

Myth: Firing black powder cartridges with an air space between the
over-powder wad and the bullet can result in a “ringed” chamber.
While buying and selling BPCRs for many years, I purchased three rifles
over the Internet in which, during initial inspection, the chambers were
found to be “ringed”.  Each had a circular or ring-shaped deformation
to the inside wall of the chamber, not a lead or fouling ring.  In each
the ring was located where the base of the bullet was positioned in the
chambered cartridge, not in front of the case mouth.  These were
clear cut examples of what can happen when using improperly loaded
smokeless ammo with an air space between the powder/wad and the
bullet base.  Until recently I believed that, if not loaded properly,
chamber rings could result when using either smokeless or black
powder.  But after additional research and checking my records, it’s
clear that two of the rifles were fired extensively with reduced
smokeless loads, not black powder as I had originally assumed.  I don’t
know what type of filler material, if any, was used and I have no idea
the type of powder or loads used in the third rifle.

In the book, The Modern Schuetzen Rifle – Second Edition (Copyright
1999) by Charles E. Dell and Wayne Schwartz, Charles Dell devoted a
chapter to his chamber ringing experiments.  Using smokeless powder,
he was successful in repeatedly ringing chambers with reduced powder
loads with gaps between the over-powder wad and bullet base.  By
holding and firing the rifle vertically to create the gap, he was also
able to produce chamber rings with reduced smokeless powder loads
without using an over-powder wad.  In a later chapter on black powder
cartridge shooting Charlie discusses his failed attempts to ring a
chamber with reduced black powder loads using the same experimental
techniques.  He wrote, “To date I cannot find any tendency at all
indicating a ringing force strong enough to damage a barrel when using
fixed ammunition loaded with FFg black powder when a gap exists
between the bullet base and over powder wad.”  By the way, ringing is
a cumulative effect and many shots are normally required before a ring
becomes apparent.  The ring may go unnoticed until case extraction
becomes a problem.

It’s a well know and accepted practice to ensure that black powder
cartridges are sufficiently filled with powder to eliminate air gaps
between the powder column, wad and bullet base.  This is certainly
the safest approach, but based on Charles Dell’s research it’s not a
critical consideration when using black powder.  Especially since he
also points out that, “Certainly our forefathers, if breech seating their
bullets, could not have followed this rule”.  To this day, some black
powder shooters that breech seat bullets routinely leave a gap
between the over powder wad and bullet base and do not have
chamber ringing problems.  I’m also aware that some breech seaters
use the same technique with smokeless powder, but based on Charlie
Dell’s findings they’re risking damaging the chamber, especially if the
rifle is old with a “softer” barrel.

Myth: Firing cartridges with cases shorter than the chamber can
result in a “ringed” chamber.
This is definitely false.  “Short-case” cartridges can have negative
effects on accuracy and create other problems, but ringing a chamber
is not one of them.  A good example is shooting .45-70 Gov.
ammunition in a .45-90 Win. chamber, a common question or concern
of new BPCR shooters or non-reloaders considering the purchase of a
rifle chambered in .45-90 Win.  Reinforcing the myth, shooters have
reported older rifles with ringed chambers, attributing the damage to
shooting short ammo in a longer chamber.

It's been my experience that what may appear to be a chamber wall
ring resulting from firing short cartridges in a longer BPCR chamber is
either hard fouling or lead deposited in front of the case mouth that
remains in the chamber after cleaning.  If moisture is trapped under
the deposits, corrosion can result.  The resulting etching/pitting can
also appear as a chamber ring.  One .45-90 Browning Creedmoor BPCR I
purchased for resale appeared at 1st glance to have a chamber ring in
front of where a 45-70 case would end.  I knew a lot of 45-70 BP ammo
had been fired in the rifle.  After close inspection with a Hawkeye bore
scope, it turned out the ring was actually lead and hard fouling.  After
cleaning out all the remaining lead with an Outers Foul Out
electrochemical cleaner and scrubbing the chamber, it was essentially
OK, but with some signs of discoloration and slight etching from
inadequate cleaning.  When peering into the chamber with a light
source at the muzzle, even after being cleaned, the slight etching
appeared to be a chamber ring.

Although not a direct comparison, I've fired thousands of hot .38-
special rounds in a .357 mag. revolver and the result was not one bit of
etching or erosion in front of the case mouth.  And there are many
"long-chambered" rifles that routinely handle BP cartridges with shorter
factory length cases with no ill effects.  Case in point is the Browning .
40-65 BPCR with a chamber length that is typically 2.125” to 2.130”.  
Many shooters are not concerned with closely matching case length to
chamber length and fire loads with 2.090” - 2.10” length cases in their
.40-65 Browning's with no resulting ringing or chamber etching.

Here’s another example.  As a youngster I fired tens of thousands of
rounds of .22 shorts and some .22 long cartridges in two different rifles
designed to handle .22 long rifle ammunition with no signs of a ring or
chamber erosion.  Whenever I could come up with a few bucks I'd pick
up a “brick” (case) of ammo and, at the time, .22 shorts were cheaper
than 22 longs or 22 long rifles.  I was taught to religiously clean the
rifle after each use until the chamber and bore were spotless.  One
rifle was a Rem. single shot and the other was a Marlin Golden 39A
lever action.  Both were designed to handle all common .22 ammo.  Of
course it may depend on the type of barrel metal used.  I would
imagine some of it can be relatively soft, especially in the cheaper .
22's.

Finally, if using a short case can result in a ringed or etched chamber,
why doesn’t a ring eventually form in the transition step/throat of a
BPCR?  Don't get me wrong. I'm not suggesting it's a good idea,
accuracy wise, to shoot “short-case” ammo.  But a rifle will not be
damaged by shooting black powder cartridges with straight-wall cases
shorter than the overall chamber length, which leads me to the next
subject.

Fact: Shooting “short-case” ammo can result in decreased accuracy.
The earlier example of firing correctly loaded .45-70 ammo in a .45-90
chambered rifle is not a safety issue and will not damage the chamber
but accuracy may be diminished.  Key to understanding the following
comments is to keep in mind that in a correctly designed chamber the
neck region is dimensionally larger than the bore to allow room for the
wall thickness of the cartridge case, and the ideal case length should
match the chamber length plus head clearance (see note below),
resulting in the case lip contacting or almost contacting the transition
step from the chamber to the throat (see note below).  Therefore,
when a cast bullet is fired from a cartridge case of the correct length,
there is very little or no space for the bullet to expand larger than the
throat and bore groove diameter.  See Figure 1.
Notes:
1) SAAMI (Sporting Arms and Manufacturers’ Institute, Inc.) defines
head clearance 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”.  In Figures 1 and 2 head clearance
is visually exaggerated for sake of clarity.  It is generally held to
0.001” for bench-rest rifles but is typically found to be 0.003” to
0.006”, possibly more, in production rifles to allow for variances in
case rim thickness.  By the way, many confuse head clearance with
headspace.  Figures 1 and 2 illustrate the difference.
2) It’s not clear in Figures 1 and 2, but my definition of the throat
includes the transition step, freebore (a smooth cylindrical section
forward of the transition step which is devoid of riflings) if it exists,
and the leade (leading edge or ramp portion of the rifling lands).
                                                           Figure 1

As the cartridge fires, internal pressure forces the case and bullet
diameters to increase.  I.e. the bullet “bumps up” (obturates)
and the case expands to tightly fit and “grab” the chamber wall.  
Experiments have shown that this happens almost instantly,
before the bullet moves forward to any extent.  As the case body
and neck expand, the case tries to shorten slightly and the case
lip tends to pull back.  At the same time, frictional forces on the
inside of the case from the powder, wad and bullet tries to
lengthen the case slightly.  Therefore, any gap between the case
lip and the transition step may or may not widen.  A full-length
resized case, being slightly smaller in diameter, will expand and
“pull back” more than a “fire-formed” un-resized case.

So let’s consider what happens when a cartridge with a
significantly shorter case is fired in a longer chamber.  A good
example is firing a .45-70 cartridge in a rifle with a .45-90
chamber.  Since .45-70 brass (typically 2.1” long) is shorter than .
45-90 brass (2.4” long), the bullet will bump up to a larger
diameter to fill the excess chamber space over 0.3” of its
length.  If there are exposed lube rings, which are typically much
smaller than 0.3” wide, the space will fill with lube and lead
from the expanded bullet.  See Figure 2.  As the expanded bullet
moves forward it is squeezed down 0.020” to 0.025” in diameter
by the transition step.  If the gap is excessively wide, as in this
example, the displaced lead may be smeared back; resulting in
“finned” driving bands or remain in the chamber as a shallow
ring.  If the transition step angle is relatively sharp (45 degrees is
common) lead may be stripped off, trapped and build up in the
transition step, which is likely to degrade the accuracy of
subsequent shots.