Bullet Setback
If you’ve been around guns for any period of time, chances
are you have heard of the term “Bullet setback.” If you haven’t, this term
simply refers to the bullet being pushed further down
in to the cartridge, thus
shortening the size of the round, or the cartridge overall length (C.O.A.L.).
Why would this be a problem? Shortening the length of the round causes a change
in the internal case volume of the cartridge, resulting in higher-than normal
pressure upon firing. In drastic cases, this rise in pressure can cause the
round to exceed the safe limits for the cartridge.
Some would use this as a talking point against reloading
your own ammunition when the truth is that it can happen with factory ammo as
well. The primary cause for bullet setback is chambering the same round
multiple times, such as in defense-carry, or conceal-carry pistols. Many times,
when storing the pistol away, the magazine is removed, the round in the chamber
is cleared and then placed back at the top of the
mag, causing it to be the
same round that is chambered the next time the gun is loaded.
Detecting Setback
For the reloading process, checking a round for setback is
simple.
One the cartridge is completed, perform a bench test, or “press test,”
and measure for setback. This is done by first measuring the length of the
round. Then place your thumb on the nose of the bullet and press the round down
into your bench and then measure the round again. If the second measurement is
shorter, your round has
setback.
A second, more definite check, would be to simply chamber
the round and measure, seeing if the length is shortened. This test, however,
is best done on a cleared round, meaning one with no primer or powder. If you
choose to use a live round, it’s best to do so at the range.
Having slight setback, slight meaning a few thousandths of
an inch, is normal. However,
is you are noticing around 0.010” of setback after
one chambering, there’s a problem in your process that needs correcting.
Causes for setback
There are several potential causes of bullet setback. If you
are experiencing it, look at one, or all, of these causes.
Sizing Die not properly set
Most cases of setback are caused when the sizing die,
particularly those for tapered rounds such as the 9mm. Since these
dies are
also tapered, they will only properly
size the round when the case is inserted
fully into the die. If too large of a gap is left between the die and the shell
holder, the cartridge will not be sized correctly and be prone to setback.
Pistol could cause it
Setback occurs when the slide pushes the round against the
feed ramp on the barrel. If you have a reloaded cartridge that passes a bench
test but gets setback when cambered, it could be a problem with the gun. To
check this, load a factory round and see if
setback occurs. If so, the problem
is with the gun. If no factory ammo is available, load the round into a
different pistol; if no setback occurs on this gun, the problem is with the
original gun.
Undersized Bullets
Though it is unlikely, your bullets could be too small in
diameter. To determine this, simply measure a batch of bullets.
These are just a few of the causes of bullet setback. If you
realize you are having an issue with setbacks, cease all reloading and
determine the problem before continuing. This will ensure your safety and
continued success.
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