TexasMac's Web Site
DETERMINING CASE LENGTHS
By Wayne McLerran
To identify the rear or starting edge of the step, a permanent marker
was used to blacken both sides of the step.  Then, using very fine
sandpaper on a flat surface, the black area on the chamber side of the
step was removed by gently rotating and sliding the case across the
paper.  The remaining edge of the black portion clearly defined the
start of the step.  Measuring the length to the start of the step results
in a close approximation of the case overall length.  Using this method
and adding head clearance, I was able to determine the case length to
within 0.005” of the actual length in my rifle.

One drawback to this method is it can be a little “tricky” to determine
the exact start of the chamber transition, but your measurement
should be close enough.  If the lands and grooves show up on the end of
the case, I do not recommend measuring them to determine bore
diameter dimensions.  “Slugging” the bore with soft lead will provide
accurate measurements of the groove and bore diameters.  Another
drawback is the technique does not take in account head clearance,
which will add typically 0.003” to 0.006”, possibly more, to the
measured length depending on the thickness of the case rims.  For a
discussion on head clearance, refer to Figure 2 and comments below.

Quick Method 2
The following technique also requires a case longer than the chamber.  
As in the “Quick Method 1” above, a full length resized .45-90 case was
used for a .40-65 chamber.  Expand and then flare the mouth of the
case so the flared portion can be felt dragging on the inside of the
chamber wall when it’s being pushed in.  Continue to push until it
comes to a hard stop, indicating the mouth or lip of the case has hit
the start of the chamber transition.  Using the rear end of vernier, dial
or digital calipers as a depth gauge, measure the amount of the case
that’s sticking out of the chamber.  Subtract the amount from the
length of the extended case.  Adding head clearance results is a very
close approximation of the maximum case length.  I found this
technique to be faster and slightly more accurate than “Quick Method
1”.
Note: Depending on the rifle, chamber transition angles can vary from
45 degrees to much less.  If the angle is very shallow, i.e. a few
degrees, this method may not work very well.  The technique also has
the same head clearance drawback as noted in “Quick Method 1”.  For
a discussion on head clearance, refer to Figure 2 and comments below.

Utilizing a Chamber Cast or Impact Impression
Chamber casts and impact impressions are excellent tools for
accurately determining correct case lengths.  Both clearly define the
starting edge of the transition step, typically resulting in a higher
degree of measurement accuracy than does Quick Method 1 and Quick
Method 2 discussed above.  For details on making a cast or impact
impression, refer to the article titled
Chamber Casts & Impact
Impressions
.

Measuring the chamber length using a sulfur or CerroSafe cast is
straight forward and simple and there’s no need to discuss it further
here.  But neither includes headspace, which is also required for
accurately determining case lengths.

Utilizing an impact impression is an excellent method for determining
case lengths.  Measuring the impression from the rear of the rim to the
start of the chamber transition includes the actual headspace of the
rifle and will provide the correct case over-all length (OAL) of a
properly trimmed fire-formed case.

Head Clearance and Headspace
Following is a short discussion on head clearance and headspace. As
defined by SAAMI (Sporting Arms and Ammunition Manufacturers’
Institute, Inc.), Head Clearance is
“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”
.  SAAMI
includes a comment that head clearance is commonly confused with
headspace, so don’t make that mistake.  Headspace is “
the distance
from the face of the closed breech of a firearm to the surface in the
chamber on which the cartridge case rests
”.  Therefore, in full
agreement with SAAMI’s definition, 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 and is clearly illustrated in Figure 2.
Updated 9/25/15

Prior to jumping into the following discussion I should make it clear
that my comments on determining case lengths are limited to rimmed
straight-wall cases that seat on the case rim when chambered.  
Similar methods are used for bottleneck rimmed cases but will not be
discussed here.

Firing a black powder cartridge (BPC) rifle with a case that’s either
too long or too short can have a direct affect on accuracy and lead to
other problems.  A
case that’s too long can result in increased
chamber pressures and will affect accuracy due to bullet deformation
and possible base fining.  A case that’s too short is the most common
situation.  One result of a short case is lead accumulating in the gap
between the case lip and the chamber transition step.  Depending on
the gap width and the position of the bullet driving bands and lube
rings in relation to the gap, the gap may fill with lube or lead from the
expanded bullet.  Assuming the gap is filled with lead, as the bullet
moves forward the
lead will either be smeared back onto the bullet,
resulting in “finned” driving bands, or stripped off and deposited in the
chamber transition step as a lead ring.  For an additional discussion on
shooting short-case cartridges go to the article titled
Shooting Short-
Case Straight-Wall Black Powder Cartridges – Myths & Facts Concerning
Chamber
Rings & Accuracy.  While reading further, keep in mind that
one of the keys to improved accuracy is minimizing bullet deformation.

The location of the chamber transition step is the key element in
determining the correct case length.  The transition step defines the
forward edge of the chamber.  It’s the short, angled region where the
chamber narrows down to the smaller groove and bore diameters of
the throat and bore.  A typical chamber transition step angle is 45
degree, but the angle can be much less for chambers designed to
handle grease groove (GG) and paper patched (PP) bullets, and is a
necessary part of accurate chamber and bore designs of cartridge
firing rifles.  It allows the chamber to accept the larger diameter of
the cartridge case and fire a smaller diameter bullet to match the
bore diameter while minimizing bullet deformation.
Note: The Miroku manufactured Browning and Winchester chambers
have a 12.5 degree transition step angle.

The ideal case length of a fire-formed (fired but not resized) case
positions the front edge (mouth or lip) of the case so that it almost
comes into contact with the rear edge of the chamber transition.  My
definition of “almost” leaves some space (not much) for case
stretching under firing conditions.  The case may initially stretch some
when fired then retract.  Therefore, if the mouth is in hard contact
with the chamber transition, “fire crimping” can result, essentially
rolling over the lip and “squeezing” the bullet to a smaller diameter as
it exits the case.  Since the relatively soft cast bullet will expand
(obturate) to fill the throat and bore, increased fining of the base
edge caused by fire crimping the case mouth may have a negative
effect on accuracy.  Once the maximum case length (mouth edge in
hard contact with the chamber transition) of a fire-formed case has
been determined, I recommend shortening the case 0.005” or so to
eliminate the possibility of fire crimping.  To determine, with a higher
degree of accuracy, how much your cases stretch, if any, when fired
will require further experimentation on your part.

So how does one determine the maximum case length?  There are five
methods that come to mind.  But regardless of the method used, an
accurate determination of case length must include the chamber
length and head clearance.  See figure 2 for an illustration of head
clearance.

Following are two quick and easy methods that result in a rough
approximation of case length, which should be sufficient for most
shooters.  The third and forth method consists of utilizes a sulfur
chamber cast or a fusible alloy chamber cast commonly referred to as
a CerroSafe cast.  The final method makes use of an impact impression
of the chamber and throat.  All will work but, as will be noted, there
are benefits and tradeoffs associated with each technique.  For details
on making a chamber cast or impact impression, refer to the article
titled
Chamber Casts & Impact Impression.

Quick Methods

Neither of the quick methods discussed below will provide diameter
dimensions of the chamber, throat, or bore.  And both techniques
require a case that is slightly longer than the chamber being
measured.  Someone that’s reforming longer and larger caliber cases
to use in a smaller caliber rifle will find the process a little easier.  
The reformed untrimmed case will generally end up longer than
required, a good thing for this exercise.  One example is reforming a .
45-70 case to fit into a .40-65 chamber.  Another example is to use a .
45-90 case for measuring a .45-70 chamber or reforming a .45-90 case
to use in a .40-65 chamber.

Quick Method 1
As already noted, this method requires a case longer than the
chamber.  A full length resized .45-90 case was used for a .40-65
chamber.  The neck of the case must be annealed to the softest
condition possible.  I stood the case up on a brick and heated it with a
propane torch until the neck area glowed red then allowed it to cool.  
To quickly cool the case and speed up the process a bit, drop it into
the water which will not harden the brass as it would steel.  Now
lightly lube the outside of the case and insert it into the chamber.

Using a wooden dowel and hammer, or similar tools, drive the case in
until the rim fully seats.  The soft brass will not damage the chamber
or bore.  Using a long wooden dowel or metal rod inserted down from
the muzzle, gently extract the case.  The location of the transition
step should be obvious but the starting edge will not be well defined.  
Refer to Figure 1 below.
Wishing you great shooting,
Wayne