Headspace Of Reloading
Brass - Part 2
In part one of our series looking at
the fundamentals of checking the headspace on reloading brass, we looked at the
reasons why headspace should always be checked. In Part two, we are going to
look at the tools necessary to check the headspace of your reloading brass
It is important to remember that
superior accuracy of reloaded ammunition its dependent on getting the headspace
of your reloading brass set correctly for
the intended rifle. You might have the headspace on your reloaded ammo set
perfectly for a rifle, attempt to use that ammo in another rifle, and your
accuracy be completely hindered. That’s why, though it is painstaking, the
reloader must set the headspace for the rifle you will be using.
to Measure Headspace
The tool to measure the headspace or
shoulder position of your reloading brass is marketed under a number of names,
including shoulder bump gauge, precision mic, or cartridge comparator, but each
does basically the same thing. They give a measurement from the base of the
cartridge to a predetermined spot on the shoulder. So, why do you need to know that?
A comparator allows the shooter or
reloader to do a number of things:
Measure the headspace of factory or
reloaded ammunition to ensure it is within spec for safe and reliable use. If
the case is too long it simply will not chamber, and if it is too short, it
will not headspace correctly, may not fire, or may result in a case separation.
Quantifies the chamber headspace in a
particular rifle by measuring the headspace of fired cases from that chamber.
After firing, a cartridge fire-forms to the size of the chamber.
It will ensure that you are getting the
minimal shoulder set-back when setting up your reloading dies.
This last one is probably the most
important for the reloader. By minimizing the set-back of the shoulder, you are
limiting the expansion or stretching of the case during firing, improving
consistency and accuracy, and extending the life of the brass. So, the
cartridge comparator also serves as a diagnostic tool to ensure the rifle
chamber is within specifications, and to gauge if the sizing die is set-up
As an example, if you measure the
shoulder on once-fired reloading brass from a rifle chamber that is reamed to
spec, you will find that the headspace will measure “0” on an RCBS Precision
Mic gauge. This is because the case has fire-formed to the length of the
New factory match ammunition may read
minus three on the same gauge. In other words, the shoulder is three
thousandths (0.003”) shorter than the spec chamber. This is so that it will
chamber correctly. After firing, that case will be fire-formed to the chamber
so should have moved the shoulder forward to match the actual size of the
Now, after you have run a fired piece
of brass through your sizing die you may find that you have pushed the shoulder
back ten to twenty thousandths (0.010” to 0.020”). This is too much. So now you
can adjust the sizing die up to bump the shoulder back just enough to chamber
in that particular rifle without any issues. Once you are getting the required
amount of bump on your bump gauge, you can lock the die.
In part three of this series, we will
look at more methods of measuring headspace, as well as some info on older
military surplus rifles.