Headspace Gauges And How To Use Them - Part I
A question that gets asked over and over on the Tech Help
Line is “What
is headspace?” and “How do I check headspace?”. Hopefully,
this article
will help answer some of those questions and provide some
useful information
on a very important aspect of gunsmithing.
Headspace is defined as the distance from the bolt or breech
face to a measuring
point within the chamber forward of the breech. With rimmed
cartridges,
such as .22 long rifle, 30-30 or .357 Magnum, the
measurement is to the
front of the rim cut. Straight- walled rimless cartridges, such
as .30 Carbine,
9mm Luger or .45 ACP, are measured to the mouth of the
chamber. Rimless
bottleneck rounds, like .223 or 30-06, are measured to a point
on the shoulder
at a certain specified diameter. Belted magnums, .300 Win.
Mag., or .375
H&H, are measured to the front of the belt cut, essentially like
a rimmed
case.
Checking headspace should be a part of any job that involves
action work,
especially if the job will require the gun be test fired.
Headspace should
be checked before any gunsmithing work is done. That way, if
an excessive
headspace condition exists, the customer can opt to have it
corrected before
expensive work is done to a gun that may not be suitable for
the intended
use. Checking the headspace first helps to protect you, the
gunsmith, from
firing an unsafe gun or from doing work that you may never be
paid for if
your customer abandons the gun to your shop.
Headspace gauges are used to check one aspect of a firearm’s
suitability
for use. If the headspace is less than minimum, factory loaded
cartridges
may not fully chamber in the firearm, the action may not close
fully, and
it may not fire. If the gun does fire, you could have excess
pressure problems,
and in the case of semi-automatic arms, possible serious
damage to the locking
system. It’s possible, with autoloading guns, to have slam fires
due to
insufficient headspace. If headspace exceeds the maximum
limit, excessive
case stretching can occur, even to the point of a case rupture
or head separation
from the case body. Other problems associated with excessive
headspace are:
failure to fire, misfires, poor accuracy, and very short case life.
With
rimfire guns and their thin brass cartridge cases, excessive
headspace can
lead to ruptures of the case at the rim, or even a complete
head separation
on firing. The sudden “dumping” of powder gases into the
action can wreck
an action or stock, and in the case of many rifles, send powder
gases back
along a bolt body to blast the shooter’s face. For this reason,
eye protection
when shooting is a must!
Modern headspace gauges are made of heat treated tool
steel. Even though
the steel is quite hard and tough, you must treat the gauges
as though they
were made of glass. If they are misused, they can break,
cause damage to
the gun being checked, or can give false readings on the
correctness or
incorrectness of a particular gun’s headspace and its suitability
for use.
Our recommendation is that you use headspace gauges in
sets from the same
manufacturer. An example of this would be GO and NO-GO
gauges from Clymer,
and not one from Clymer and the other from Forster. Your
results will be
more uniform if you stick to this recommendation rather than
mixing different
brands.
Under normal commercial circumstances, a gun whose bolt
closes completely
on a NO-GO headspace gauge (even if it does not close on a
FIELD gauge)
should not be put out for sale or returned to a customer as
being suitable
for use, until the headspace situation is corrected. Keep in
mind that a
firearm may have perfect headspace, but may have other
chamber problems
or action problems that could keep it from being used safely.
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| Dykem Steel Blue Layout
Fluid |
There are normally three types
of gauges made;
GO, NO-GO and FIELD. The GO gauge corresponds to the
SAAMI (Sporting Arms
& Ammunition Manufacturer’s Institute) minimum chamber
length, while the
FIELD gauge usually matches the maximum chamber depth, or
slightly less.
NO-GO gauges are an intermediate length between minimum
and maximum, that,
technically, is a voluntary dimension. A firearm that closes on a
NO-GO
gauge and does not close on a FIELD gauge may not give
good accuracy and
may have very short cartridge case life from the ammunition
re-loader’s
standpoint.
Case head separations
are a possibility with a gun that closes on a NO-GO gauge. It
may also
fail to fire or suffer from misfires, depending on the firing pin
dimensions.
Here’s a very thorough list of instructions on how to use
headspace gauges
to check headspace. It’s broken down by type of firearm and
type of actions
so you can hone in on the kind of firearm that you work on
most.
Bolt Action Rifles
The most common usage of headspace gauges is with
bottleneck cartridges
in bolt action rifles. As with any job, check the chamber and
magazine
to be certain the rifle is not loaded. Remove the bolt from the
action,
and strip the bolt as far as practical. This means remove the
firing pin
assembly and, if possible, the extractor. If the bolt has a
spring- loaded
plunger in the bolt head as an ejector, remove the ejector.
Clean the
chamber, the bolt locking lug recesses in the action, the bolt
face, locking
lugs and the gauge. With some types of bolts, the extractor
can’t be removed
without damage to it or its retainer. For many years,
Remington centerfire
rifles have used a rivet to retain the extractor. Do not remove
the extractor
on Remington rifles.
Open and close the stripped bolt on an empty chamber
several times to
get a feel for the amount of resistance the bolt has to closing.
Slip
the GO gauge into the chamber and with the pressure of only
one finger,
attempt to close the bolt. If you are working with a
Remington rifle,
or any rifle where the extractor cannot be readily removed,
slip the rim
of the gauge under the extractor on the bolt, and push the
bolt and gauge
forward to chamber the gauge. Attempt to close the bolt
handle. IMPORTANT:
If you feel any resistance to closing greater than you had
with the chamber
empty, stop right there. Don’t force the bolt closed. The bolt
handle
gives you a tremendous amount of leverage, so to avoid
damage to the gauge
or rifle, use only minimal pressure to close the bolt. If the bolt
closes
with no feel to the handle with a GO gauge in the chamber,
open the bolt,
remove the GO gauge, and replace it with a NO-GO gauge.
Again, gently
attempt to close the bolt. The bolt handle should stop or
show resistance
to closing at some point before it fully closes. DO NOT force
the bolt
handle closed. Even if the bolt handle closes 98% before you
feel resistance,
the headspace is normally considered to be within specs.
If the bolt handle closes fully on the NO-GO gauge, repeat
the test with
a FIELD gauge. If the bolt does not close completely with this
gauge,
the headspace is on the long side, but the rifle can usually
still be
used with factory ammo, if the cases will not be reloaded and
there are
no other problems present. Never fire a gun that closes on a
FIELD gauge.
If it is fired, the chances are extremely high that you will get
case
ruptures.
.22 Rimfire Headspace
Modern .22 Rimfire cartridges fall into two families: .22 long
rifle,
.22 long, .22 short, BB and CB caps; and .22 Winchester
Rimfire and .22
Winchester Rimfire Magnum. The .22 long rifle family shares
case body
diameters, rim diameters and thickness. The overall cartridge
length,
bullet shape and weight are the variable factors. The .22 WRF
and .22
WRFM differ in body length, and the magnum is loaded to
much higher chamber
pressures.
 |
Small
step at arrow
shows amount
of headspace (+.005”) with a Clymer
GO gauge placed in the chamber
of a .45 ACP, 1911 Auto barrel |
.22 rimfire cartridges present
several potential
problems to the gunsmith. Most bolt action rifles, as well as
lever action
rifles, pumps, semi-auto rifles and pistols have a barrel breech
that is
essentially flat, and use a recess in the bolt head to regulate
headspace.
Bolt, pump and lever action rifles have a locked breechbolt
system, while
autoloading guns are usually blowback operated, with the bolt
held forward
by spring pressure. If you only measure the depth of the bolt
face rim cut,
you will not get a true picture of the headspace situation, as
the bolt
on a locked breech system usually has some clearance built
into the bolt
face to barrel breech face dimension. In addition, autoloading
guns will
build up dirt, powder fouling and bullet lube on the face of the
bolt and
the breech end of the barrel, which will temporarily increase
headspace
by holding the bolt back slightly from the barrel.
Revolver Headspace
The vast majority of revolvers are chambered for rimmed
cartridges. They
fall into two general types of actions: swing out cylinder and
solid frame
revolvers. Modern Smith & Wesson revolvers are swing out
cylinder types,
while Ruger Single Action revolvers have solid frames, where
the cylinder
pin has to be pulled forward to remove the cylinder for
cleaning. In addition,
there are two types of cylinders: rebated and non-rebated.
Rebated type cylinders have a recess for the case rim and
come almost to
the frame at the rear of the cylinder. Non-rebated cylinders,
when viewed
from the side, leave the entire thickness of the case rim
exposed.
When checking revolvers with swing out cylinders, start by
cleaning the
cylinder, including the chamber, the ejector star and its recess
in the
cylinder, the front face of the cylinder, the barrel breech and
rear of
the frame window where the firing pin comes through. On
rebated rim swing
out cylinders, put a feeler gauge between the cylinder and
frame at the
top rear of the frame window to check clearance at each
chamber. Note these
measurements. Open the cylinder and place the GO gauge in a
chamber. The
GO gauge should be flush or just below flush with the rear
face of the cylinder.
If it protrudes above the cylinder’s rear face, measure how
much it sticks
up. It should be less than the feeler gauge thickness that you
noted earlier.
Next, place the NO-GO gauge in each chamber. It should
protrude enough that
the cylinder cannot be closed with very gentle pressure.
Caution: Forcing the cylinder closed with a headspace gauge in a
chamber may damage
the cylinder, the ejector, the revolver’s frame or the headspace
gauge.
The gauge is heat treated harder than any of the gun’s
components and can
break if mistreated.
With a non-rebated cylinder, start by cleaning the gun as
detailed above.
The cylinder should close easily, with no drag on the gauge,
with a GO gauge
in the chambers. If the headspace is correct, the cylinder will
not close
with a NO-GO gauge in each chamber. Do Not force the
cylinder closed with
any gauge in a chamber.
Single action, solid frame revolvers have their own
peculiarities. The area
surrounding the firing pin in the frame is usually raised above
the surface
at the rear of the frame window. With the cleaned revolver on
half-cock
(or in the case of a current production Ruger, with the loading
gate open)
slip the GO gauge in the chamber and slowly rotate the
cylinder. The gauge
should rotate past the raised section with no drag. Next, place
the NO-GO
gauge in the chamber and repeat the test. The gauge should
stop the cylinder’s
rotation. Carefully back rotate the cylinder as far as possible,
remove
the cylinder pin from the frame, and remove the cylinder from
the frame.
Replace the cylinder in the frame and repeat the test on the
other chambers.
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| Match Rifle Headspace
Gauges |
Shotguns
Bolt action, pump and autoloading shotguns can generally be
checked and
treated like their centerfire rifle equivalents. Break open guns,
whether
side by side, over and under, or single shot types, must be
checked carefully
to avoid damage to the gun. Start by checking the headspace
with the barrel
off of the action. The back surface of the GO gauge should be
level with
or drop below the level of the rim recess in the barrel being
checked, while
the NO-GO gauge should protrude above the level of the
recess. After checking
with the barrel assembly off of the action, reinstall the barrel
assembly.
Place the NO-GO gauge in the chamber being checked. Very
carefully and slowly
attempt to close the barrel. DO NOT “snap” or force the action
closed. The
barrel assembly on a break open type gun can exert
tremendous leverage upon
closing. Snapping the barrel closed with a hardened gauge in
the chamber
can cause damage to the gauge, barrel and action.
This is as complete a rundown on headspace gauges and
headspace as I can
give you. If you want more information on headspace or on
specific chamber
dimensions, a great source is SAAMI. They publish a manual of
chamber prints
that lists headspace and complete dimensions. SAAMI can be
contacted at
(203) 426-4358. Headspace is one area of gunsmithing it pays
to be knowledgeble
and cautious about. Remember, headspace is where a very
small error can
lead to a really big problem.
Tools Used In This Article
| #262-100-004 |
Dykem Layout Fluid |
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| #079-308-000 |
Brownells .308 Match Rifle Headspace Gauge
Kit
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