The return of the giant
.357 Headspace Part I
By Joseph D'Alessandro Editor
what you'd call a new gun specialist. Fundamentally, that means I buy guns
chambered for new cartridges, gripe about not being able to get ammo, dies
and handloading data, then lose interest as soon as those problems no
longer exist - only because I have nothing left to complain about.
I first purchased my P229, the .357 SIG cartridge had been out there for
awhile, but people were still deeply embroiled in the mind numbing
"locate on the case mouth or locate at the shoulder" debate. In fact,
straight case handgun guys were intimating the SIG round was the work of
the devil, with possible ties to the late Elmer Keith.
writers suggested reloading the 357 SIG cartridge was best reserved for seasoned
and skilled handloaders, indicating extraordinary steps had to be taken
with handloads to ensure the gun and cartridge would fire reliably. Still,
just about everyone continues to agree, this is a terrific round with lots of
power and accuracy, with a high degree of performance potential.
my initial experience with the 357 SIG I didn't have a major problem with factory ammo, although
some of it appeared to be dimensionally marginal, but I did have a
heck of a time with handloads. The
symptoms pointed to excessive headspace, but I couldn't locate a proper
set of headspace gauges to help isolate the problem. I eventually found a way to use the barrel of
the P229 as a fixture for measuring relative headspace as documented in
the original article which is
still plopping around Real Guns at http://www.realguns.com/archives/001.htm.
As a result of this experiment, I arrived at several conclusions -
The case must headspace on
The shoulder must be
located approximately .649" from the case head
I needed set my sizing die
in a non-standard position to properly locate the case shoulder
I accepted these basic propositions, all of my problems disappeared, and
the P229 became one of my favorite pistols - so I locked it away and lost
interest, at least until now. I was sitting at my desk, pretending to be
working, but really just paging through the Brownells catalog, knowing there
must be some gadgetry I needed to buy. Fortunately, newly listed go/no-go gauges for the
357 SIG jumped right out at me.
Headspace gauges typically
exist as a three piece set: a "go" gauge, where the gauge is cut to the minimum
SAMMI chamber length, a "no-go" where the gauge is cut to
slightly more than the
maximum SAMMI chamber length and finally a way oversized "field" gauge.
A action of a firearm in good condition should easily close over the go
gauge, will just barely not close over the no-go gauge and won't come
close to ingesting the field gauge. If the
firearm's action closes over a field gauge, the gun is in serious need of repair
and should not be fired. Since I knew I didn't
have a problem with too small of a chamber, I placed an order for
a "no-go" gauge from Brownells.
At $25, the gauge seemed an inexpensive and useful tool to verify safe gun
specs for the present, and potentially verify a new barrel fit sometime in
What is this thing
called, headspace ?
Headspace is the distance
between the point of the chamber that prevents forward movement of the
cartridge, and the breech or recoil face of the firearm. If you compared
this dimension to the SAMMI or other industry standard for the cartridge,
you will know if your gun's headspace is excessive, below minimum, or
within the allowable range. An
acceptable level of headspace permits any cartridge produced within
designed tolerance, factory or handload, to chamber and
When headspace falls below the
minimum, it may be difficult to chamber a round and close the breech. If the bolt were forced closed,
and the forward end of the chamber jammed the case into the bullet, this
condition could create unsafe levels of chamber pressure when the gun is
discharged. Of course, not being able to chamber a round when anything big,
hairy, and with claws is upset, might generate
a certain amount of anxiety.
Excessive headspace permits a
fired case to be driven back against the breech face at a speed equal to
the bullet's velocity. The more space, the more travel, the more travel,
the greater the damage to the pressurized case. Under extreme excessive
headspace conditions, the unsupported case area might rupture, or the firing
pin might not be able to reach the too distant cartridge with enough force
to adequately strike the primer. In any case, headspace is something to
always keep under control.
you see a Winchester 125 grain 357 SIG practice round and the Dave Manson
Loon Lake precision 357 SIG no-go gauge. Since this critical gauge is
obviously designed to
locate at the shoulder, I believe we have our answer to the question, "Does the 357 SIG
headspace on the shoulder, or the case mouth ?".
headspace in a firearm is typically .006" greater than the maximum
length of a standard cartridge or, in the case of the 357 SIG,
0.865"+.006" = 0.871. However, the 357 SIG cartridge is
positioned at the shoulder, so I was looking for a maximum dimension from
shoulder to case head of 0.649"+.006" or .655. The no-go gauge
measured .659" or .010" beyond maximum case length. One quick note, there is a notch in the rim of the gauge.
Because of the increased rim thickness of the gauge, the extractor won't clear
the rim without a notch. If the extractor is too wide to clear the
notch, the extractor needs to be removed before checking with the a
go/no-go gauge. In fact, anything that would interfere with the placement
of the gauge, chamber or the recoil surface of the slide should be removed
to prevent an erroneous reading.
one time, recoil operated semi-automatics were not generally considered to
be as headspace critical as a bolt action rifle. With
the increase in high pressure pistol cartridges moving what was once 18,000 PSI
chamber pressure to upwards of 40,000 PSI, perhaps headspace has become
progressively more critical. In addition, increased law enforcement use of
these type of pistols makes reliability an absolute requirement.
With the no-go gauge installed
and indexed to clear the extractor, the SIG came close to closing, but it
did not. The idea is not to slam the gun closed, but to ease the slide
down on the gauge. Unfortunately, even with the slide held measurably
open, the trigger could
be pulled, and the hammer dropped. That can't be good. As a result of this
check, I do have a pretty good idea the gun is within chamber spec, but
there are no dimensions scribed on headspace gauges, so I couldn't
directly verify correct headspace through measurement.
is actually the only reason I had for buying the SIG. I got so tired of
pulling recoil springs and plugs out of the ceiling and walls, I bought
the SIG just for the take down lever feature, and gave away all of my 1911
When it became obvious the
gauge could demonstrate the gun was sound, but not provide the details of
immediately set out to see if I could use the no-go gauge in other
ways. I popped open the gun and set up my dial indicator, then collected
some: loaded ammo, handloaded ammo, fired brass and resized brass to try
to better understand the dimensional relationship of all of the above.
last time I ran this setup, I made the assumption that the barrel
tang was the closest surface to the recoil face of the slide, so by
placing the barrel in a vertical position under a dial indicator, and
measuring the difference between the height of the tang and the height of
the case rim, I could detect excessive headspace - The height of the rim
would be more than .006" below the rear surface of the barrel tang. I
used my RCBS case checker, which is certainly accurate enough for this
I took a look at my original
article in the Real Guns archive and realized that reliable ignition came
with positive readings (rim above the barrel tang), and misfires at
relatively minor negative readings (rim below the tang) by as little as
-.004". Cases formed with an RCBS sizing die properly installed in my
Ammo Master, all had excessive shoulder set back, resulting in negative
readings. At the time, all factory Remington ammo ran between
.005" and .010" above the barrel tang, Speer between -.005"
and +.012" .
This time, when I ran the
check with the dial indicator, the results were quite different. L to R -
Empty - tang position used
to set zero on the dial indicator .000"
No-go gauge +.005"
Winchester white box ammo
Remington yellow box
Fired empty +.005"
Sized empty .000"
No-go gauge, verify gauge
The negative measurements for
Remington and Winchester factory ammo concerned me, and I will spend some
time at the range investigating if these short rounds will yield the same
results as the last time around - misfires. The resized case measurement
threw me because I couldn't get anything that wasn't short from the Ammo
Master, yet the Rock Chucker was popping out near perfectly dimensioned
brass. The only difference between the two presses that would impact
measurement are the shell plate and the shell holder. I need to
investigate further and pinpoint the cause. The fired case measured
identical to the no-go gauge, but I'm not sure at this point I understand
I'm going to take a break
while I'm waiting for a go-gauge and a new depth micrometer to show up
with UPS. I want to run some of the Remington and Winchester brass through
the P229, and I need to fish around and verify some of the chamber
dimensions so I can be accurate in determining where all of this
dimensional instability is coming from. I'll be back. I may be learning
something about shell holders / plates, practice grade ammo vs. premium
ammo and a pistol's dimensional tolerance.