Some Random Thoughts on Headspace, Headspace
Measurement and Gauges, Case Measurement and Bullet Seating
There has been a lot of
discussion and expressed misunderstanding about Headspace and its measurement
and importance on various web discussion pages.
For the definitive work on
headspace and all that goes along with it, check out the writings of P.O.
Ackley.
For another view on headspace,
read on.
An unfired round of ammunition,
of a certain caliber, is constructed to dimensions determined by the manufacturer.
All ammunition of that caliber, constructed by that manufacturer, will be
identical within that manufacture's production limits. When a round is fired in
a rifle, the case will expand, in all dimensions, and fill the chamber, -or-,
split open before reaching the chamber constraints. Assuming that the case does
not split and is somewhat ductile, as brass is, the case will assume chamber
dimensions, less about .001 inch in all dimensions due to springback
of the metal. This fired case will have "zero" headspace, for all
practical proposes, in that chamber.
The difference in
dimensions between the unfired case and the fired case are what is important.
In fact, for rimless cases that
are headspaced by the shoulder and the head of the
case, only the change in length, from the shoulder to the base, is important.
The lengthening of the case at the neck is of no importance, nor is the
increase in diameter.
It is the
"stretching" of the case lengthwise that is important, and is the
main reason for "headspace gauges", and the need to measure
headspace.
Ammunition manufacturers and
rifle makers have decided that only a certain amount of stretch is allowable,
somewhere in the range of 7 to 12 thousandths of an inch, any more than that
and you run the risk of destroying the casing.
Headspace gauges are made to
compare the actual chamber dimensions with the dimensions set by the
manufacturer of the ammunition.
It is very important that
the headspace gauge match the AMMUNITION that is to be fired, not the
chamber.
Certain people are claiming
that special "old style" or "old dimensioned" gauges should
be used to evaluate military rifle chambers. This is only correct if the
ammunition to be used is constructed to the "old" dimensional
specifications. Most ammunition, even that made 30 years ago, is
constructed to current specifications, and that means that current
specification gauges should be used.
The only safe way to determine
if a certain rifle/ammunition combination will be safe is to measure an unfired
case and then measure that case after firing. This means that a case-measuring
tool is required, such as an RCBS case micrometer. If the case stretches by
more then 10 or 12 thousandths of an inch, then the
rifle is probably unsafe to fire.
If you are a reloader, and use fired cases from a certain rifle, then
you have complete control of headspace for that rifle!!!!! Remember, that once
fired in a chamber, the case will have perfect headspace. All that you have to
do is resize the case without changing the shoulder-to-base dimension. You
don't have to use neck-size dies in order to achieve this, just don't,
never, ever screw the re-sizing down against the shell holder,
like the manufacturer says. Invest in a case micrometer, and adjust the die
downward, re-sizing and measuring, until the micrometer shows that you have
shortened the case one-thousandth of an inch, and you are ready to go. I AM NOT
TALKING ABOUT CASE TO MOUTH MEASUREMENTS. INVEST IN A CASE MICROMETER AND STOP
TRIMMING CASES TO LENGTH. YOU WILL GET BETTER CASE LIFE AND HAVE MORE TIME FOR
SHOOTING!!!!!
A case length micrometer (RCBS)
is shown below.
On the right is a round of
30-06 ammunition loaded at the Lake City Army Ammunition Plant in 1967 for use
in the National Matches. I have coated the bullet and case shoulder with blue
paint, the stuff used to lay-out machine work. Never fear, you will not use blue
this stuff. Next to the ammo, at the bottom, is a micrometer base unit for the
30-06 design, above it is the case length portion of the micrometer. The long
white and black bullet shaped thing is a collapsible dummy bullet, and on the
right is a loaded bullet length micrometer piece. With the exception of the
loaded ammo, everything comes as a unit from RCBS.
The RCBS micrometer provides
two distinctly different measurements, first, case length - measured from the
case base to the imaginary line on the case neck, and second, overall bullet
length - measured from the case base to the place where the bullet diameter
matches the bore diameter.
Above is the loaded 30-06 round
in the micrometer. Although the micrometer can be used for loaded ammo, it's
most useful for unloaded or fired cases. See that the indicated case length is
reading "0". This means that the case, headspace wise, exactly
matches the industry standard set for the 30-06 case. The graduations are in
0.001 inches (Thousandths)
Above is the loaded round being
measures for base-to-rifling engagement length. The reading is 0.143 inches.
Look at this picture very carefully ! When a case is designed the distance from the
base to "somewhere" on the shoulder is specified. IT IS THIS DISTANCE
THAT SETS THE HEADSPACE SPECIFICATION ! The micrometer
has left a mark (line), visible above the leftmost arrow, and has compared the
distance from the case base-to-this line with the case designers
specification and found it to have a variation of "0". Remember the
"0" reading above ?
NOW, LISTEN UP. If you were to
measure cases for headspace length and then fire them and re-measure, you could
determine the chamber condition as to headspace. BUT MORE IMPORTANTLY, when
resizing fired cases for use in the same rifle you should only re-size to about
0.001 less that the fired length !
Additionally, look above the
right arrow. Here is the line left by the micrometer when it measured the
distance from the case base to the place on the projectile where the projectile
engages the rifling. This distance won't mean much to you unless you have first
inserted the collapsible dummy bullet in you rifle chamber, closed the bolt,
(which collapses the dummy), and then measured the dummy with the bullet
portion of the micrometer.
Most reloaders
agree that the loaded and chambered ammo should place the bullet close to the
beginning of the rifling, say within 0.020 inches and many times closer. This
means that you need to find out where the rifling begins IN YOUR RIFLE and then
set your bullet seating die so that the bullets end up in the right place. This
can be done with the dummy and micrometer.
Steve Wagner
Bellwood PA
stevewag@aol.com