some understanding of measuring techniques you can do a pretty fine job of
determining what the throat to breech face measurement is, what the overall
cartridge length is, how to seat to this measurement, and how to adjust your
size die... all based on simple measurements and the threads per inch on your
Take the barrel off the frame first!
If you drop a sized case into the chamber, its head should be flush or protrude
no more than a couple thousandths. If it does not, you best learn now how to set
a size die, because you are absolutely correct about these break open guns being
different from bolt guns. This same case with a bullet seated way out can be
dropped into the chamber, leaving the head sticking out the end a measurable
amount. Measure this distance the case protrudes with your caliper or a depth
mike if you have one or access to one.
The difference between protrusions is the amount you need to seat the bullet
deeper to get to "0." You will then know exactly what the overall length is. You
can seat the bullet down to get "0" protrusion, or simply subtract the amount of
protrusion from the round's present overall length to arrive at the distance
from throat to breech face for that given bullet. If you measure the actual gap
between your barrel and breech face with feeler gauges, you can get it down to
the thousandth of an inch.
You don't need any gauges or gizmos to do the job. Most of these gizmos are like
fishing lures designed to catch fishermen and make a buck rather than catch
While on the subject... you have a micrometer on your seating die and did not
Follow this. Let's use an RCBS seat die for an example with a 1/4-28 thread.
100 divided by 28 = .0357. So one full turn of this 28 thread per inch seat stem
equals .0357" of movement up or down.
Half of this is .0179," half of this or 1/4th turn is .009," and half of
this or 1/8th turn is .0045." Follow the logic? While you cannot readily
get it down to the last thousandth of an inch, you can get mighty close,
normally plenty close to the order of magnitude needed to be of practical
value. Do the same thing with any one's seat stem, no matter what the
thread pitch is. Just divide the number of threads per inch into 1 inch to
find out the value of each full turn.
You can "eyeball" a reference point, or make marks on the die itself. I just
note the position of the stem and turn it however many turns or fraction of a
turn I need to.
If you cannot determine exactly how many thread per inch are on the stem, you
can buy a little thread pitch gauge from Brownell's or one of the tool supply
houses. Or you can hold the stem against a ruler/tape measure and count the
threads in one inch as close as you can. So far as I know, all seat dies use
standard thread forms, usually National Fine thread. Look at any chart of thread
sizes and find the one with the diameter and threads per inch closest to what
This same principle is quite handy for adjusting size dies also. You have a 14
pitch thread on the die body. That is .071" per turn. Following the logic
outlined above, you can with a little practice get your size die adjustments
down into the order of magnitude necessary to stay within the normal .006"
maximum headspace allowance.
For example, you performed "The Experiment Every Contender Shooter Should
Perform" on my web site and found that you need to bump fired case shoulders
back about .010" to get to "0" headspace. 1/8th turn is .009," rounded off, so
that would leave you just .001" headspace... perfecto. No jamming the case head
forward or up when you close the barrel.