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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 casings.

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.

Tools 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:

1. 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.

2. 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.

3. 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 correctly.

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 chamber.

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 chamber.

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.