View Full Version : Checking .30 Carbine wear on muzzle and chamber

February 14, 2008, 04:36 PM
How does a Go / No-Go gauge work? I have not seen one, and from what I think I understand it is basically a solid steel blank shaped like a shell. One is the correct size, and one is too big. If the too big shell closes inside the action, then the headspace is too worn. Like wise, if the correct size closes in the action, you know the chamber is not too small. Do I have this correct?
Secondly, I have seen gauges which fit down into the muzzle to check wear. What is acceptable wear at the muzzle, and what sucks? Will a gauge such as one designed for a .30 Carbine also work for a M1A, and a M1 Garand?
Where is the best place to get these gauges? I looked in Brownells, and did not find them. I looked in the index under gauges. Nothing under muzzle or bore. Nothing under go / no-go. What pages in catalog # 60? Thanks guys. I am just trying to learn some of this stuff myself.

James K
February 14, 2008, 05:37 PM
I have an older Brownells catalog (57) and it shows .30 carbine in both Forster and Clymer - the former has GO, NO-GO and Field, the latter has only the GO and NO-GO.

There is not a lot of call for those for several reasons. One is that the carbine is lower pressure than the .30-'06 and so the locking lugs and lug seats don't get battered as much, plus both rebarrelling and making up of rifles in that caliber is uncommon. Because the U.S. carbine cartridge was non-corrosive from day one, barrels usually stayed in pretty good shape, so bad barrels on carbines are uncommon. That situation also meant that not a lot of barrel scrubbing was needed so the muzzles didn't get the cleaning rod wear that the M1 and M1903 rifles got. Muzzle wear on the carbine is also uncommon. It can, of course, be measured with the same gauge as for any .30 bore.

Here is something on headspace, primarily oriented toward the 7.62 NATO, but applicable to all other cartridge firearms:

To begin with, the "head" of a cartridge is its base or back end. That's why the markings on the back of the cartridge case are called the "headstamp".

So, headspace is simply the space for the "head" of the cartridge. In a rimmed cartridge, this is obvious, but for all cartridges, it really is a measurement of the room for a cartridge from the bolt face to whatever stops and supports it in the chamber. For rimmed cartridges, that is the front of the rim; for belted cartridges, it is the front of the belt. For cartridges like the .308, measurement is taken from a specified point on the shoulder; for a cartridge like the .45 ACP, the measurement is from a sharp shoulder which abuts the case mouth. So we say that a .308 headspaces on its shoulder, and that a .30 Carbine or .45 ACP headspaces on its case mouth. For our purpose here, we will assume that the gun is a rifle in .308 Winchester, but we need to know that headspace is a factor in pistols and revolvers as well.

Some headspace is absolutely necessary; if no tolerance is allowed, operation of the rifle may be difficult or impossible. But while there is a correct range, headspace can be wrong in either direction. If there is insufficient headspace, a cartridge will either be difficult to chamber or will not chamber at all. In combat, this could spell disaster more certainly than excessive headspace.

What problems can result from excessive headspace? The answer is in what happens when a rifle cartridge is fired. The front of the cartridge case is made thin, because it needs to expand to seal the chamber and prevent high pressure gas from coming backward. But that thinness means that under pressure the case will grip the chamber walls very tightly. The rear of the case, being thicker, will not expand, and the pressure will push it backward as far as it can until the breechblock or bolt stops it. The case will stretch. It is nearly impossible to prevent some case stretching; if the gun is to operate normally, there must be some play between the bolt and its locking mechanism. But if the stretching is such that it exceeds the elastic limits of the case material, the case will tear apart. At best, this will leave the front part of the case in the chamber and hang up the gun. At worst, high-pressure gas will be released into the system and possibly damage the gun or injure the shooter.

Some folks confuse excess headspace with an oversize chamber, and think that excess headspace can be handled by reloading without full length resizing of cases. That is true if the case has simply expanded into an overlarge chamber, as it will do if the case head is held by the extractor. Even if headspace is excessive, and the breechblock can actually back up, neck sizing can delay the inevitable, but not prevent it. The condition will worsen with firing until no care in reloading can compensate, the case head will protrude too far out of the chamber, and the case will bulge and blow out, with the pressure release wrecking the rifle and possibly injuring the shooter. No one should be deluded into the belief that excess headspace is not dangerous, or that reloading techniques will correct it.

Why are measurements needed? Why are two measurements necessary? Why not make every chamber of every gun to the exact dimensions required?

The answer involves the nature of machine work. Chambers are reamed with a tool called (surprise!) a reamer. If only one rifle were to be made, it would be possible to make a reamer to the exact dimensions and it would cut an exact chamber. But in mass production, it doesn't work that way. The designer of a cartridge specifies certain tolerances, based on his knowledge and, to some extent, the anticipated use. When a reamer is made to cut chambers for that cartridge, the reamer is made to the outside tolerance, or the largest allowable size. As chambers are cut, the reamer wears, and when it becomes dull, it is sharpened. This continues until the chamber is at the smallest allowable point, when the reamer is discarded and a new one used.

This system introduces one element of variation in cartridge chambering. The other is simple wear. When a rifle fires, the pressure generated inside the cartridge case pushes back the case, which then pushes back the bolt, which then pushes on the locking seats in the receiver. After a while, the bolt lugs and the receiver wear enough from this pressure, combined with the friction of normal operation, that the bolt can move more than desirable under pressure, and we say that headspace has become excessive.

Now, remember that reamer that was used to cut chambers? Well, it is not the only reamer involved. Reamers also cut the chambers on tools used to manufacture ammunition, and they are used and sharpened the same way, so the size of the ammunition can vary. Reloaders use sizing dies that are also made by reamers, and those reamers are made and used the same way. In factory production ammunition is made to tolerances, so some cartridges may be said to be "long" and others "short" even in the same batch.

Now, when a rifle barrel is made it is either not chambered at all, or given a "short" chamber. Unchambered barrels are used by gunsmiths to build rifles for custom cartridges. Short chambered barrels are used where the final caliber is known, but it is desirable to adjust headspace after installation of the barrel and selection of a bolt. Two gauges (or gages) are used at the factory and by gunsmiths to ensure that the chamber and bolt are within specifications for the cartridge. These are called the "GO" and "NO-GO" gauges. Their use must be understood in terms of the tolerances of the cartridges that the rifle will use.

The GO gauge ensures that the rifle will close and operate with the longest cartridge that is within tolerances for the ammunition. The NO-GO gauge ensures that the shortest cartridge that is within tolerances will not be allowed to stretch far enough to exceed the elastic limits of the case material.

But we mentioned that normal use of the rifle will cause changes in the dimensions of the locking system and the locking seat(s) in the receiver. That fact led to the development of a simple "one gauge" test to ensure that the rifle has not become dangerous. This test is by use of a FIELD gauge. A rifle that accepts a FIELD gauge may be nearing, at, or past the danger point; the only way to know which is by knowledge of that rifle, or by the "feel" of the gauge. At best, failure of the FIELD gauge test delivers a warning, like the wear ridges on tires. At worst, it signals certain danger. Even a rifle that fails the FIELD gauge test may function normally with cartridges at the long end of the cartridge tolerance, yet be dangerous with cartridges at the short end.

The term "FIELD gauge" should not be taken to mean "the field" in a military sense. No one calls "time out" in battle to check soldiers' rifles with a FIELD gauge. In this sense, FIELD simply means any place outside the factory, such as a depot or an arms room.

Another point of concern is how long a normal rifle will last, in terms of rounds fired, before headspace needs to be checked. For most shooters, the answer is, "Don't worry about it." The fact is that most rifle owners will never live long enough to see their rifles develop excess headspace. But in military service, especially in "familiarization" firing, rifles wear out rapidly, and headspace checks are routinely carried out. Match shooters too, who often fire tens of thousands of rounds a year, will check headspace every few months.

In most cases, headspace should be checked every five thousand rounds, just to be on the safe side. But the reality is that barrels will usually wear out before headspace becomes a problem, and many match rifles have had several barrel replacements with the same receiver and bolt. Since a new barrel will be final chambered on the rifle, the headspace will always be reset at the time of barrel replacement. If bolt or receiver wear makes it impossible to obtain proper headspace, the worn part is scrapped.


February 15, 2008, 10:42 AM
Jim, thanks for the short post.;) Tons of great info.
I do still have a question on erosion of the muzzle. I have seen pictures of gauges stuck into the business end of carbines starting at zero and up to 2.0? This is a gauge of how much wear has been made to the muzzle from what I understand to be cleanig and shooting. Could you expand a little on this process of measuring this wear, and its significance? What number is acceptable, what is worn out? At what point is it a sign that you may want to have a new crown re-cut? Or is the deciding factor only a function of how the rifle shoots?

James K
February 15, 2008, 03:02 PM
It sounds like you have had more experience with rifles like the M1 and M1903 than with the carbine. On sites dedicated to those rifles, people seem to never shoot the guns, they spend 24/7/365 checking them with gauges.

Just to summarize the problem. If the inside of the barrel at the muzzle is badly worn, the lands will be worn away unevenly, or the bore may even be worn to an oval shape. The result is that when the bullet exits, one side is free first, and the gas escaping past that point tips the bullet to the opposite side. Since the effect is pretty unpredictable, the bullets will take off in different directions and accuracy is non-existent.

The carbine, like the M1 rifle, was cleaned in service only from the muzzle, so the steel GI rod could wear the muzzle even though the steel of the rod is softer than the steel of the barrel itself. But because GI and US commercial carbine ammo was/is non-corrosive, the need for cleaning, and hence the muzzle wear, was considerably reduced. Muzzle wear is not caused by erosion, and the word does not really apply. It can be caused by rust if the barrel has been exposed to salt water or something similar that will actually eat away the steel.

While a MW gauge is good, the old bullet trick works about as well. Simply take a GI .30-'06 cartridge and stick the bullet in the muzzle, point in. If there is a gap of 1/8" or more between the muzzle and the neck of the case, things are OK. When MW is very bad, in fact, you don't really need a gauge; you can look at the ends of the lands and see the wear and flattening.

Re-crowning will not solve the problem of excessive MW unless you cut off a 1/2 inch or more. What can be done is to counterbore the muzzle, which means drilling the end of the barrel out to a larger size to clean up the ends of the rifling lands. That too, can have its problems, but can at least restore some accuracy to a badly worn barrel.

What I am trying to say is that if you are dealing only with the M1 carbine, I doubt that either muzzle wear or headspace is a problem. Your best bet might be just to shoot the gun and see how accurate it is.


February 15, 2008, 05:25 PM
Thanks Jim. I totally understand your statement about guys playing more with the gauges, than shooting the things. I picked up a DCM Garand in like 1993, and I just got a couple M1 Carbines from the CMP. I had a competant Garand gunsmith go over the M!, and replace any worn springs, etc. On the Carbine, I have just been reading a lot online, and doing things myself. It is getting apparent that due to the nature of the 30.06 vs. the .30 cal carbine round, I may be being overly cautious. I just like to know what I am doing, and if I don't know, I don't mind asking someone who does. I appreciate you takiing the time to answer my posts. It is very nice of you. BTW, I ordered a bunch of .30 cal. carbine, and M1 ball ammo from the CMP, so I definetly will be shooting my milsurps. Have a good weekend.