View Full Version : Why you should never buy headspace gauges

Brian in MN
September 05, 2001, 08:54
I received my new Forster headspace gauges yesterday. I bought the GO & NO GO gauges. After opening the package I whipped open the safe & started hauling out .308's. The results were pretty F'ing depressing. Of the nine rifles I tested only two behaved properly. One was too tight and the rest closed on the NO GO with ease.

Among those that were too loose:

New National Arms Argie
and a Ruger M-77 that I bought new. It has had about 170 rounds through it.

Guess I'll be hunting up a field gauge.

September 05, 2001, 09:04
Use GP's field expediant method of masking tape on the end of the nogo for .005 more to make it a field gauge.

September 05, 2001, 11:48
Brian in MN

I received my new Forster headspace gauges about a week ago. I got the GO & NO GO gauges also!

FWIW I tried to use them to set headspace on a kit and could only find about .0010" difference between them!

I'm going to bring them to work this week and check the dimensions! I think they may be made wrong :eek: !

I don't want to believe it, but it's the only thing I can think of that would cause a .0010" difference between the NO and the GO(using gage pins)!


Timber Wolf
September 05, 2001, 16:14
Don't you measure gauges from the base to some place on the shoulder? How can this be done accurately?

50 cal
September 05, 2001, 17:17
Headspace is measured to the datum line on the shoulder. Unless you know where that line is, it is a wasted effort to try to measure the gauges.

I've used headspace gauges to buy used rifles at gun shows. Stick that no go in there and if it closes, make a small scene about it so no one else will want it. You can get bolt guns for just the actions that way. I've done it a bunch of times to ge the actions to build up new heavy barrel bolt guns.

September 05, 2001, 18:09
Remember that .308 and 7.62 NATO aren't exactly the same length. The 7.62 NATO is a few thousandths longer, so a .308 headspace guage may not have the proper dimensions.

September 05, 2001, 18:42
The Forster gauges are not very well thought of in some circles. Go over to Culvers Shooting Page and look up Lane's CSP tips. Gus Fisher, former USMC match armorer recommends Clymer and has little good to say about the Forsters. Can't remember what his gripe was, look it up.

WJ-Polish Guy
September 05, 2001, 19:24
Hehe.. you all dudes make me laugh, sticking those saami gauges into Fals.
I bet poor Dieudene could not even pronaunce "saami", nider he gave shit `bout it.... :D

Educate yourself both on FAL factory spec and 7.62specs...
There is field gauge avail from Cylmer to check Fals... Go to the Crufler, read his reviev of the gauge, also some basic info in that reviev... :cool:

September 05, 2001, 20:09
Originally posted by Timber Wolf:
<STRONG>Don't you measure gauges from the base to some place on the shoulder? How can this be done accurately?</STRONG>

Timber Wolf

The Measurement comes from a point on the taper which measures .4000 Dia. to the Base. I've got access to an Optical Comparator at work that will go up to 100 times Magnification with a Digital Readout that goes to .0000"! I can't hardly believe that they're wrong, but I have a suspicion they are, or of course I could be wrong!

Like I said I tried to headspace a kit and found that it only took a gage pin difference of .0010" between the Go and the No Go! Maybe I'm missing something, but I checked it 2 or 3 times and then tried the old Masking tape on the GO(I checked it .005 thick) and I could see about the same difference on the gage pins!

FWIW I'll know tomorrow!

Brian in MN
September 06, 2001, 08:57
Thanks for pointing me at the Cruffler review, WJ. I was hoping it was a .308-7.62NATO thing giving me grief.

R4 fan
September 06, 2001, 11:37
Here's the info from Cruffler.com if you can't locate it... ;)

Headspace dimensions on rifles chambered for the 7.62mm NATO cartridge can be misleading. As a case in point, military FAL headspace dimensions are not identical to those of the dimensionally similar .308 Winchester cartridge. For comparison's sake, a rifle chambered for the .308 Winchester should have a minimum chamber length of 1.630", which equates to the .308 Winchester "GO" headspace gauge. A new rifle chambered for the .308 Winchester should have a headspace dimension no larger than 1.634" - the same length as the .308 Winchester "NO-GO" headspace gauge. Finally, a .308 Winchester rifle that has seen use should be considered unserviceable if the chamber dimension is larger than 1.638" - the length of a "FIELD" headspace gauge. The FAL, on the other hand, has a minimum chamber length of 1.6315" and a maximum chamber length of 1.640."

For some strange reason, FN didn't see it necessary to consult the US based Sporting Arms and Ammunition Manufacturer's Institute (SAAMI) when determining headspace dimensions for the FAL. As a result, it is very possible, when using solely .308 Winchester gauges, to make the determination that an FAL has excessive headspace when in fact it is perfectly serviceable FOR THE MILITARY 7.62MM NATO AMMUNITION FOR WHICH IT WAS DESIGNED. We added emphasis for a very good reason. If you choose to fire commercial .308 Winchester in an FAL, then the .308 headspace specifications are controlling. In other words, an FAL with a headspace dimension of 1.639" is considered to be unserviceable and unsafe with .308 Winchester ammunition, but perfectly serviceable with 7.62mm NATO military ammunition.

Now we come to the real issue - since most commercially available headspace gauges for the .308 Winchester include a maximum length of only 1.638", how does an FAL enthusiast know whether his rifle is within specification for military 7.62mm NATO ammunition? The answer can be had in the pages of the Clymer Manufacturing Company's catalogue. Located in Rochester Hills, Michigan, the Clymer Manufacturing Company was started in 1958, and continues to provide high quality gun tools to both the professional gunsmith and hobbyist. Among Clymer's products are chambering and finishing reamers as well as headspace gauges.

For the FAL-minded among you, Clymer also offers a.308 Winchester "Field" headspace gauge of 1.640" in length. FAL enthusiasts that we are, we immediately ordered this tool, so that our FAL gauge set would be "complete." It arrived a few days later. The gauge is finished bright, with only the barest evidence of machining marks on the tapered portion of the gauge. It is carefully marked:

CLYMER 998 (this is a date code meaning that the gauge was made in September, 1998)
308 WIN

The gauge boasts a wide, well cut extraction cannelure and a deep firing pin recess. There is also a cut-out in the rim. While this was originally done so that the gauge could be driven on a cylindrical grinder (an "O.D" grinder), it neatly doubles as an ejector cut out for rifles with plunger type ejectors. We found the cut out to be too narrow to permit the extractor on an FAL to slip through it.

The gauge is as well made and finished as any tool we've seen. It fit smoothly into the chambers of three different FAL's, an M1A, and a commercial .308 Winchester hunting rifle with no hint of binding. We immersed the tool in solvent for 24 hours to see if the markings would disappear. They didn't. All in all, we were well pleased with the gauge.

If you are an FAL hobbyist, enthusiast or gunsmith, we believe that at $25.00, the Clymer .308 WIN "Field" gauge is a must have item. However, we again caution the reader to remember that headspace specifications are for a gun/ammunition combination. Just because the factory specification for FAL headspace is 1.640" maximum, this does not mean that an FAL which will not close on a 1.640" gauge is safe to fire with commercial .308 Winchester ammunition. Be sure of both your gun and the ammunition for which it is chambered. Be safe. After all, you wouldn't be investing in gauges if safety wasn't a prime concern!

And here's Clymer's website...


September 06, 2001, 11:45
Here's a few numbers to chew on.

1.630 (.308 Win. minimum headspace)
1.632 (7.62 NATO minumum headspace)
1.634 (Forster NO-Go)
1.636 (Clymer NO-Go)
1.638 (Forster Field)
1.640 (FN Field)

September 06, 2001, 12:57
To confuse the issue even more, I submit the following

(Revised 04-17-01)

Headspace is the distance between the front of the bolt face and the back of the brass casing when the casing is as far forward in the chamber as it can go. When the round fires, the chambered casing moves backwards until stopped by the bolt face. The chamber and the bolt enclose the casing during firing and contain the massive pressures from the burning propellant. If the headspace is over the "safe" range, these pressures are not adequately contained and catastrophic failure may result. If the headspace is under the safe range, the chambered round swages into the throat, increasing pressures dramatically. The round could also fail to chamber, and a round with a high primer may fire before the bolt locks.

The way you measure headspace depends on the caliber. You measure rimless rounds from the back of the case to the case mouth. Use the belt on belted magnum cartridges. You measure center-fire cartridges that have shoulders from the back to a specified point on the shoulder. In the case of .308 Winchester, it is the point on the shoulder where the diameter is 0.400." You can get by without headspace gauges, although this is one area where you may not want to take short-cuts.

The three gauges are called GO (1.630"), NO GO (1.634"), and FIELD (1.638"). A GO gauge is the minimum chamber dimension. The NO GO gauge is the maximum chamber dimension. A FIELD gauge is the "super maximum" or the point where it is totally unsafe to fire the weapon even under combat conditions. Dimensions are measured from the rear of the gauge to the point on the shoulder where the diameter is .400" Many rifles with NO GO readings can still be safely fired, but one would not leave the arsenal in that condition. With respect to FALs, we are in luck. The allowable variance in headspace is much greater than in other rifles.

Most rifle bolts have lugs on the front that rotate or cam into place, thereby "locking" the bolt. The FAL is different in that the bolt does not rotate, but instead moves forward and cams down into lock. The locking surfaces are not rotary lugs, but the lug on the bottom rear of the bolt. This engages a part in the receiver called a locking shoulder. The locking shoulder is that cross piece that sits on top of the part that holds the magazine catch, bolt hold-open, and ejector. It is called the ejector block.

The proper way to set headspace on a FAL involves putting a GO headspace gauge in the chamber, inserting a specially calibrated tapered rod in the locking shoulder hole, and pushing the stripped bolt and bolt slide closed. Read the number point on the tapered rod that just allows the bolt to close and select a correspondingly sized locking shoulder from your vast inventory.

There are 16+ sizes of locking shoulder, but you have to make do with the one that came with your kit. I have found that locking shoulders measuring .262" are the largest commonly needed and can be ground to size. Strip the extractor and firing pin assembly leaving just the bolt in the bolt carrier. Install your GO gauge in the chamber. Gently push the bolt carrier forward. It should close completely. If not you have "short" headspace, then take your NO GO gauge and repeat this step. The bolt should not close, even with slight pressure (don't slam it). If a GO gauge closes and a NO GO does not, your rifle is properly headspaced. Mark the bolt and bolt carrier with the rifle serial number. If you plan on having a spare bolt or carrier, verify these as well and mark with the rifle serial number. Bolts and bolt carriers are NOT always interchangeable.

You don't want to buy a gauge? You really should. Get a factory loaded Federal 168g or 173g National Match .308 cartridge or unfired brass. This is your GO gauge. Cut a disk of masking tape or a feeler gauge that fits over the base of the cartridge. The case plus tape is your NOGO gauge (that's right -- the thickness of a piece of tape is the only variance allowed). You must not use anything but National Match ammunition. Typical military 7.62x51mm is slightly undersize for reliable feeding in belt-fed machine-guns that can handle the variances.

Some locking shoulders are larger diameter for installation into worn holes. If you try to put one of these into a new receiver, you'll have problems. If its all you have, you can carefully reduce the diameter of the pin with #220 sandpaper. First reverse the locking shoulder and test fit the leg into the receiver recess - it may require light sanding, but is usually a tight press fit. DSA receivers are not cut as deeply here so a small amount of the locking shoulder leg will protrude. This is not a problem.. Then reverse the locking shoulder and test fit from the left side of the receiver to make sure you do not have one with an oversize shank. If it will start into the hole, drift it back out and install. If it is too large, lightly sand with #220. Ideally, you will use a micrometer to check the shank and compare your reading to the table at the end of this workbook.

The locking shoulder can be installed with an arbor press if you can properly support the receiver. It is easier to hammer it in. The leg is brittle so you must be careful. Use hammer with a brass or aluminum bunch. Be careful to only the part over the shaft to avoid breaking the leg off. I recommend using a large a large roll pin punch for those locking shoulders with a hole on either

When you drive it in, the leg might turn slightly and be out of alignment - grasp with small wrench and turn. If using a crescent wrench, insure the adjustment screw doesn't drag across the receiver. It should be initially hard, then easy, then it sometimes bottoms out on the edge of the far hole and needs to be tapped in from the front, then driven the rest of the way. Occasionally a shoulder requires a heavy hand to get it in, but usually just moderate strokes. A press is nice, but offers the possibility of bending the receiver if something slips out of alignment.

A .308 headspace dimensions are 1.630" minimum (GO) and 1.634" maximum (NO GO). The difference is .004" which creates a problem. The FAL must have interchangeable parts. The weapon needs to be factory assembled without hand-fitting parts. The locking shoulder moves slightly ("setback") during initial firing. These variations are cumulative. The amount of tolerance needed follows.

Interchangeability of parts 0.0045"
Headspacing 0.0010"
Locking shoulder set back (.0007 to .0022) 0.0015"
Factory tolerance 0.0070"
Field wear tolerance 0.0015"
Total headspace 0.0085"

Tolerances are the sum total of the maximum error in each of seven categories. I doubt any weapon would display maximum variance in each category, but it is theoretically possible. Our acceptable variance is now 0.0085." This is far greater than the .0040" variances of our gauge. Because of this, the Rifle Steering Committee ordered extensive evaluation of tolerance variations and safe operating dimensions and came to the following conclusions.

"A" dimension is from the .400 point on the cartridge shoulder to the bolt face (headspace). "B" is from the .400 datum to the locking shoulder face.

U. S. GO and NOGO gauge A 1.6300" to 1.6340" (.0040")

Factory new rifle, test fired A 1.6315" to 1.6385" (.0070")
B 5.4475" to 5.4500"

Rifle in Service A 1.6315" to 1.6400" (.0085")
B 5.4475" to 5.4515"

Given the predictable wear of parts and the locking shoulder set back, I like to headspace all barrels to .001" under where I want to end up, which is .002" over minimum, or half way between a GO and a NO GO. I do not recommend setting for minimum headspace. I find the popular press extolled virtues of a "minimum" headspace to be highly overrated, and the increased reliability of slightly looser headspace to be valuable, considering the wide range of quality in the available surplus ammunition market. Consider this. The Gunwriter-Whores* say selecting a service rifle with minimum headspace is a good thing. But what chamber reamer cuts the smallest chamber? That's right - the oldest, most worn one. So a new razor sharp reamer will cut a perfect chamber, and a dull, worn-out reamer will cut a sloppy chamber, perhaps with chatter marks and scoring, but it will be "tighter." I'm not impressed. By the way - my "loose chamber" philosophy is for service grade autoloaders, not precision bolt guns or rifles restricted to special ammunition.

*A "Gunwriter Whore" is someone who writes glowing reviews of every free gun he gets, to insure he gets more and to insure the manufacturers continue to spend big bucks on advertising in their magazines. Every gun reviewed is the best he's ever seen. Gunwriter Whores use terms such as "new gun syndrome" to rationalize favorable reviews on guns that won't function out-of-the-box and they try to convince their readers that every gun should require several hundred rounds of "breaking-in" or tuning by a master gunsmith before it can be expected to perform its basic function of going "BANG!" every time the trigger is pressed.

Typical 3M brand masking tape is 0.005" in thickness. Therefore, if your bolt closes on a NOGO gauge (1.6340"), you are still okay. If it closes on a NOGO plus a piece of masking tape (1.6390"), you are unsafe (FIELD is 1.638"). If it closes on your field-expedient gauge made with a National Match Cartridge (we are assuming a National Match case is the 1.6300" GO minimum) and a piece of tape (approximately 1.635"), then you are okay, just on the long side of acceptable. Two pieces of tape (approximately 1.640") are unsafe. Remember that these measurements are for the FAL only and do not apply to other rifles.

Short headspace is usually characterized by difficulty chambering. If you have a .308 finishing reamer and a non-chrome-lined barrel, you can cut the chamber deeper until you reach the proper dimensions. You can also remove material from the bolt locking surface, however I do not recommend this because it precludes you from using a different bolt. The best way is to get a proper sized locking shoulder.

Remove the locking shoulder with a #7 (7/32") roll pin punch. Using a flat head punch invites slipping and scratching the receiver. You may be able to file the locking surface, but in all but the hardest files, the metal will be too hard. Use the edge of a thin sharpening stone or 220 grit sandpaper to remove material from the face of the locking shoulder. Follow the original angle and remember that you are removing less than the thickness of a piece of masking tape between test fits. The edge between the top of the locking shoulder and the face must be beveled, so if you remove enough material to make this edge sharp again, sand it back down. Reinstall the locking shoulder using a brass hammer or your pin punch. Insure you align the oval tab with its recess. The slightest misalignment will make this brittle tab break. You can turn it with a wrench to align it prior to seating it in place.

This method may require removing, fitting and reinstalling the locking shoulder several times. A way to get it right the first time is to go to a machinery supply store and purchase steel dowel pins. They come in .0005" increments on the expensive ones and .001" on the cheap ones. They are about $4 each and you can get away with every other pin if you don't want to buy the whole set. Figure you will need .256" to .264." Insert the dowel in the locking shoulder hole with a GO headspace gauge in the chamber. Find the pin that just barely allows the bolt to close. Hone your locking shoulder to the same dimension.

Excess headspace means upon firing, the web of the cartridge case is not supported by the chamber, This causes the brass to expand at the web, which weakens it. Extraction can then result in the base of the case being ripped off. You can fix by finding a bolt and bolt carrier combination that fits, but you will probably need to purchase a new locking shoulder. I have found that locking shoulders measuring 0.262" to the locking face are large enough for most guns. Take this new locking shoulder and follow the directions for short headspace rifles.

If your bolt will not drop into place, but will close with moderate pressure, you may not want to risk breaking the locking shoulder by removing it again. Apply liberal amounts of lapping compound to the bolt locking surface and the locking shoulder, and with a case in the chamber (no extractor) slam the bolt home repeatedly. Note that I wrote "a case" and not "a live round." Do not use your gauge.

Go to the range and fire your completely assembled single shot rifle with liberal amounts of lapping compound and with the gas at the lowest setting (0 or 1) until the parts lap into place and make it a self-loader again. Usually 10 - 20 rounds. Use only U.S. factory ammo for this "shooting in.". A high primer on a reloaded round, in conjunction with short headspace, can cause it to fire before it is fully chambered (out-of-battery) with catastrophic results. Do not fire unless the bolt closes all the way. Usually, the force of the bolt snapping forward from its fully retracted position is enough, while the cycling action of a fired round is not. If a fired case sticks in the chamber, do not stomp on the cocking slide handle. Hold the handle and slam the butt of the rifle on the ground.

Every once in a while, the question pops up about the differences between7.62x51mm NATO and .308 Winchester. For most purposes, the two calibers are interchangeable, however for the advanced home gunsmith, there are some differences which deserve mention. We must differentiate between chamber dimensions and cartridge case dimensions (Jerry Kuhnhausen, et al.). It is important to note that Fabrique Nationale Herstal, DGFM-FMAP Argentina, and Imbel Brazil all stamp their FAL rifles designated for civilian sales as ".308 Win." This should be enough evidence to establish the interchangeability of the cartridges in the FAL rifle, however for those using non-military components, the following elaboration may be of value.

The commercial .308 Winchester and 7.62x51 NATO cartridges are both based on the T65 experimental military cartridge. The .308 Win was standardized in1952. NATO standardized the 7.62x51mm in 1954. Both .308 Winchester and 7.62x51mm NATO chamber headspace are referenced from the point on the 40 degree shoulder of .400" diameter (the .400" datum line). Therefore your headspace gauges should be the same for both. Although some industry drawings combine the and lists the cartridge headspace as being 1.627" MIN (GO) and 1.634" MAX (NOGO) the .308 Winchester chamber headspace dimensions are 1.6300" MIN (G0), 1.6340" MAX (NOGO), and 1.6380" REJECT (FIELD). The 7.62x51 NATO minimum chamber headspace specification (specifically for the US M14 rifle) is 1.6355 +.0005" and the average 7.62x51 NATO cartridge is on average 1.632" +. The point is an average 7.62x51mm NATO cartridge is .002" + longer than a minimum commercial .308 Winchester chamber. There is a theoretical possibility that a high primer, stuck firing pin or other problem combined with a long cartridge in a short chamber could allow the weapon to fire without the cartridge being fully locked in place (out-of-battery firing) by the camming action of the bolt. The easy solution is to headspace at 1.632" or ideally at 1.6315" so the inevitable setback of the locking shoulder on initial firing will leave you with a 1.632" headspace. This allows for reliable use with the widest variety of ammunition.

On the subject of ammunition, it is also important to consider that no manipulations of chamber dimensions will compensate for the erratic performance of low-quality ammunition. I have been happy with Hertenberger, Portuguese, most Israeli (some ruptured cases), South African (hard primers), Malaysian, British (Radway Green), Czech, USGI, and US commercial. I have not been happy with Indian, Chinese, 1950s FN, or South American.

Timber Wolf
September 06, 2001, 20:44
I do not have the book in front of me but Brownell's lists .308 "match" gauges in increments from small to big. .223 same thing as I remember. Can't remember the manufacturer either.

September 06, 2001, 21:22
Well here I was already to prove something(to myself ;))! So here's what I got on the ol Optical Comparator a work today;

Forster 1.6300 GO Gage checked a Dia of .4006 at the Gage line!

Forster 1.6340 NO GO Gage checked a Dia of .4009 at the Gage line!

WTF! They both looked pretty good! I still didn't believe it so when I got home I grabbed a barrel, put it in the vise, and grabbed my big ol Mag Base with a Indicator!

Dropped in the Go Gage, set Zero!

Removed Go and Inserted No Go Gage dropped Indicator, and what do you know a .0035" difference(Go Figure)!

So I don't know what I was doing wrong the other day :confused: ! I must have had a piece of $hit on the Go Gage!

Dazed a Confused!

September 06, 2001, 22:15
Originally posted by Timber Wolf:
<STRONG>I do not have the book in front of me but Brownell's lists .308 "match" gauges in increments from small to big. .223 same thing as I remember. Can't remember the manufacturer either.</STRONG>

The set sold by Brownells is by Forster.

The range is 1.630 to 1.638.
Go is 1.630
No-Go is 1.634
Field is 1.638

Mind you, these dimensions are about .002" shorter respectively than the dimensions typically applied to FAL's.

Bottom line seems to be, if you are somewhere between 1.630 and 1.640 you are probably going to be fine.

September 07, 2001, 02:29
Let's take another look at these numbers.

1.630" Forster GO
1.630" Winchester .308 minimum
1.6315" NATO 7.62 minimum
1.632" Clymer GO
1.634" Forster NO-GO
1.636" Clymer NO-GO
1.638" Forster FIELD
1.638" Winchester .308 maximum
1.640" Clymer FIELD
1.640" NATO 7.62 maximum

In my opinion the well-equipped FAL armorer (is that YOU?) would have a set each of Forster and Clymer GO and NO-GO gauges; this way you could more accurately measure your actual headspace. (If you're going to keep these rifles for a lifetime, doesn't it make sense to keep on top of things yourself?)

I would use the Clymer NO-GO as a FIELD, for I personally would never allow any of my rifles to exceed 1.636" even if shooting only NATO surplus. Actually, the day I could chamber a Forster NO-GO, but still not a Clymer NO-GO, would be the day I'd probably set that rifle aside until I could change out the locking shoulder. Call me a safety fanatic, but either family of ammo has a LOT of power and I happen to like my face the way it is.

As most often is the case, Gunplumber said it best:
"The easy solution is to headspace at 1.632" or ideally at 1.6315" so the inevitable setback of the locking shoulder on initial firing will leave you with a 1.632" headspace. This allows for reliable use with the widest variety of ammunition. (emphasis added)


[ September 07, 2001: Message edited by: Radio ]

September 07, 2001, 11:55
Hey Dean D - can you please explain the difference between a .308 win GAUGE (not chamber or cartridge) and a 7.62 gauge? Some of us self-appointed experts would like to know.

And yes, its a trick question . . ..

April 10, 2007, 17:01
As with locking shoulders, this issue is often and needlessly over complicated.

April 11, 2007, 10:33
I agree with the issue being overly complicated.

The most important tool to me is really a case gauge. That tells you exactly what you headspace is by measuring fired cases. I made my own headspace gauges and the .400" datum line on the shoulder is impossible to measure without a case gauge. I use the headspace gauges to get me close when building and then measure the fired cases and adjust headspace to what I want based upon what the cases are telling me. 308 and 7.62 are effectively the same case size. The 7.62 headspace dimension is generous to make sure the ammunition will chamber in a dirty gun and the cases are thick enough to stretch to the larger headspace dimension without rupturing. That's why many 7.62 guns close on a No Go gauge, which is normally fine as long as you fire 7.62 not 308 commercial.