The makeup of most brass casings is actually roughly 70%
copper and 30% Zinc (the exact combination will differr between manufacturers and
is often proprietary.) The addition of zinc into the mix allows the composition
to be softened or hardened easily by working the material to enhance its
performance. In fact, most brass casings are both soft and hard.
The head of the case, the section where the primer is
inserted, is hardened to retain its shape while under pressure. The neck and
shoulder section of the case is softer to allow for more movement. This is done
intentionally to allow the neck to have the holding power for the bullet, but to
be flexible when the round is fired. When that occurs, the neck expands against
the chamber, releasing the bullet and sealing the rear of the chamber while
under pressure and releasing one the pressure escapes the end of the barrel.
When the brass retracts, it’s shape has been altered to fit that of the gun’s
chamber; this is called fireforming.
Fireforming is used by reloaders to essentially create a
different round than the factory round that was fired. Gunsmiths can ream a
gun’s chamber to allow for increased pressure, which leads to higher velocities
and increased accuracy. Fireforming after a chamber has been reamed molds the
brass to the chamber’s new dimensions. This has even led to new factory rounds,
such as the .300 Weatherby and .280 Ackley Improved.
One of the biggest concerns, when it comes to fireforming,
is maintaining proper headspace; the measurement of the case from the rear of
the bullet to the primer hole. Reaming the chamber can cause the fired case to
shorten from the force of the firing pin shoving the case into the end of the
chamber, thus decreasing the headspace. This is a problem more so with rimless
cartridges than rimmed or belted cases.
A “rookie mistake” is intentionally using a smaller powder
charge to fireform a case in an attempt to save money. The problem with this
approach is that the pressure, then, does not reach the necessary point to
fully push the brass against the side of the chamber, hence not creating a true
form. The best ways to save money is to use a jacketed bullet with a full-power
load to ensure you get a true fireformed case.
If you are truly in dire need of saving money, some
reloaders will simply not put a bullet in the round, creating a “dummy” round.
This can be done by using a substance, such as uncooked cream of wheat cereal,
with a fast burning powder, topped off with a piece of cotton to hold the round
together. The cereal will allow the powder to fire just as if a bullet was in
the round and create a true form to the brass.
When fireforming, it is best to use new brass that has never
been fired. If that isn’t an option, use once-fired brass. The reason behind
this is simple; the more times a case is fired, the thinner the brass gets. If
the brass is already thin when it is fireformed, doing so can cause cracks and
other catastrophic failures in the case.
This approach should only be done with the utmost attention
to safety. Never, under any circumstance, attempt to ream the chamber unless
you are a fully trained gunsmith. Any error could cause irreparable damage to
the gun and potentially harm the shooter.