How do you headspace a semi-auto?


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ocabj
January 24, 2005, 10:50 AM
I understand how to headspace a bolt action, but how exactly do you headspace a semi-auto? The concept behind headspacing a bolt action is that the bolt should close on a go gauge and not on a no-go or field gauge, and this is easy to determine. But how are you supposed to see if the bolt on a semi-auto closed or didn't close properly to such minute degrees?

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Dave Sample
January 24, 2005, 01:43 PM
I wouldn't touch this thread with a 10 foot pole! I have only done a few hundred 1911's so what would I know?

R.H. Lee
January 24, 2005, 01:52 PM
Here's a thread discussing headspace in semi autos:
http://www.thehighroad.org/showthread.php?t=114095

Jim K
January 24, 2005, 03:54 PM
The same principle applies to handguns, though there is no Field gauge for handguns. You use a set of GO/NO-GO gauges. On a recoil operated pistol, you can tell if the gun locks up properly on the GO and if it doesn't lock up on the NO-GO. Just remove the recoil spring and the extractor before using the gauges.

On a blowback pistol, you can tell pretty easily if the slide closes fully or not, and the same rules apply. On a revolver, you tell by whether the cylinder closes or turns freely with the GO and not with the NO-GO.

A small amount of excess headspace in a pistol is usually not as critical as in a rifle so far as safety is concerned, but too tight headspace can prevent some cartridges from chambering properly with potentially disastrous consequences in a defense handgun. Extreme headspace can result in burst cases in the same way as in a rifle.

Jim

ocabj
January 24, 2005, 04:12 PM
I was specifically interested in the headspacing of semi-auto rifles such as the Garand. I was reading the thread that addressed 1911s and explained that a 1911 shouldn't go into battery on a no-go gauge. Does this also apply to a semi-auto rifle? I'm assuming that a rifle won't operate the sear and drop the hammer if it's out of battery, so is that the way you would check to see if the semi-auto's bolt is in battery on a go/no-go gauge?

Jim Watson
January 24, 2005, 04:46 PM
Probably the same way you do a bolt action.
Strip the bolt and see if the bare bolt will close on the gauge with light finger pressure. The camming action of a bolt action or the recoil spring of an auto will ram it shut on a field gauge in a perfect chamber, causing any rifle to fail the check. Hatcher's Notebook has a long discussion of that on surplus 1917s, but it would apply to about any rifle.

1911Tuner
January 24, 2005, 05:45 PM
ARRRRRRRGGHHHHHHHHHH!!!!

:rolleyes:

At great risk....Jim Keenan explained it as it should be. (Jim...Yer a brave fellow.) The procedure for checking headspace on a weapon varies, depending on whether you're building one using a semi-finished chamber, or simply verifying headspace on one that's already in service. On the custom rifle or autopistol, the smith has a little control, and can set the headspace as short or as long as he likes...within safe parameters, of course. On an existing firearm, it's checked using SAAMI standard GO/NO GO gauges...with an extra field gauge for bottleneck rifle cartridges that headspace on the shoulder.

Pistols that headspace on the case mouth have no field gauge specified, and use the 2-piece GO and NO GO set...unless some enterprising young lad with time on his hands and a lathe at his disposal decides to make a field gauge to
check for wear or for other reasons. :p

Other cartridges that headspace on different places have different gauges...
Examples are revolver and rifle cartridges that headspace on the rim...like the .357 Magnum and the .45-70 Government... and belted rifle cartridges that headspace on the belt.

Regardless of the cartridge/ weapon design, the purpose of checking and verifying headspace is mainly for reasons of safety, though generally speaking,
the closer that you can get to ideal, or zero headspace, the easier it is to
wring top accuracy out of the gun. Limiting factors here are feed and return to battery reliability...in which the design of the gun plays a role. A semi-auto
needs a little positive headspace...or play..in order to chamber easily. Bolt-action rifles offer greater leverage with their powerful camming action, and can actually be set with negative headspace...in which the round is a little too tight, and is forced into battery by the manually-operated bolt. This also has its limits, because if the headspace is too tight, the case can be deformed in the chamber, which can not only be detrimental to accuracy, but can also drive pressures off the scale.

Many handloaders "adjust" their headspace for bolt-action rifles by fire-forming their brass, and neck-sizing only, so that the case will fit the chamber tightly. This is a practice not recommended for gas-operated
and lever-action rifles that don't offer the chambering leverage of the bolt-guns.

Here endeth the lesson.

Luck!

1911Tuner
January 24, 2005, 06:05 PM
Now then...Setting headspace on a semi-auto rifle is another lesson, and is accomplished in a couple different ways.

A military armorer has one rifle on the bench with a good barrel, and 500 bolts
at his disposal can select-fit a bolt to provide acceptable headspace using the gauges.

An armorer or riflesmith who has only one bolt must set the barrel back on the chamber end and re-ream the chamber. Either method will also work for bolt and lever-action rifles.

Rifles with chrome lined chambers...such as the M-14 and Springfield M-1A
don't lend themselves to cutting the chamber face back on a lathe, and are generally headspaced using select-fit bolts.

Dave Sample
January 24, 2005, 09:49 PM
No Comment. (and you can quote me) woof woof woof.....................

Jim K
January 25, 2005, 08:37 PM
Hi, Tuner,

Permit me a couple of comments. The question of whether a field gauge is used has nothing to do with whether the cartridge headspaces on the case mouth, it has more to do with the power of the cartridge and how likely it is that a used rifle will have enough wear to be concerned. But the only rifle cartridge I can find that does headspace on the case mouth is the .30 carbine, and there is a field guage for it.

Pistols generally have a lower working pressure (I am referring to conventional pistol rounds, not "pistols" that fire .50 BMG or the like) so those gauge sets don't include the field gauge.

As to setting the headspace to absolute minimum, that may be OK if the only ammo used will be carefully selected or hand loaded. But if the user may need or want to fire factory ammo, there is a chance that a given round will not fit the chamber. No big problem with a special range rifle, but not good for a police or military rifle.

At the risk of being extra long winded, I will post a summation that has appeared elsewhere. See the next post.

Jim

Jim K
January 25, 2005, 08:41 PM
To begin with, the "head" of a cartridge is its base or back end. (This is not the same as having one's head up... never mind.) 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 rimless 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 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 .45 headspaces on its case mouth. For our purpose here, we will assume that the gun is a rifle, 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 point, 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.

Why are there 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 chambers. 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 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.

When a rifle barrel is made it is either not chambered at all, or given a "short" chamber. The former barrels are often bought by gunsmiths to be used to build rifles for custom cartridges. The latter 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 or 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, the FIELD gauge 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.

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.

Jim

Dave Sample
January 25, 2005, 09:28 PM
Woweeeeee!

N.M. Edmands
January 25, 2005, 09:37 PM
Dave, do you think that just maybe ,you bring some of the poo on yourself? :uhoh:

1911Tuner
January 25, 2005, 09:44 PM
Excellent post Jim! Thanks for the time and effort that it took.

You made an interesting point on the rifle analogy in that the lugs and recesses can become worn...or battered by overpressured ammunition...to the point of creating an excessive headspace condition. That point can also be applied to our favorite service pistol as well.

Due to dimensional issues, some M-1911 barrel/slide combinations effect what is known as "Slap Seating" in which a lug or lugs are actually set back on firing. This was once a practice with softer barrel steels that would allow
the lugs to "Equalize" when fired a couple of times with proof loads...or a
couple hundred times with standard ammunition. Oc course, this setback,
or slap seating was limited to a thousandth of an inch or two...but it did create a like amount of extra headspace that the pistol didn't have until it was accomplished.

This brings us back to the point of the chamber stop shoulder to hood face
being correct...but still allow an excessive headspace condtion due to the
specs and tolerances in the barrel to slide lug fit. This can occur even in a pistol with minimum headspace.

When the locked-breech pistol is fired, the bullet friction and pressure drives the barrel forward, while the slide...and the boltface (or breechface) is driven
to the rear. Whatever the amount of play is in the lug fit...a like amount of headspace will occur. This is what makes it possible...over time and due to wear and/or abuse...to create an excessive headspace condtion in a pistol that began life with safe, acceptable headspace. OR...You can simply use a
file to thin the front faces of the barrel lugs by .015 inch or so and create the condition in about 10 minutes. If the gun was set up with .010 inch of headspace when new...setting the lug faces back by .015 will absolutely
take the gun out of spec, and dangerously I might add.

This is what I suspect had happened with the famed Norinco barrel that got all this hoopla started. The fronts of the lugs were battered and set back due to an unlock/linkdown timing problem with the gun, and when the lugs were dressed and cleaned up, they were about .012 inch or so narrower than they were when new...and the reduction was in the critical location on the front faces of the lugs....allowing gap between barrel hood and breechface to widen when the gun was fired. Given the fairly loose tolerances present in the majority of Norincos, this caused the barrel to fail miserably on the NO GO check in a known good Colt slide...and even worse in two Norinco slides. This, even though the chamber to hood face dimension was pretty close to ideal.

Dave Sample
January 25, 2005, 10:31 PM
N.M. Edmands: Perhaps you are right. But I think that no talent people like to take shots at me because of their lack of a life. I think it's criminal now to be good at what you do if you work with your hands and mind. I knew more 50 years ago about 1911's than most of the people here do now. When I was a Navy Ordnanceman on the Flight Decks, I also had to take care of the small arms along with the 20mm Cannons. The pilots carried 38 Specials M&P's and Lamma .380's, plus I had to be able to make 1911's run. I have been carrying 1911's for over 50 years, have built them for 20 years, am an expert on creating Custom Caspians from parts, was doing that long before Ed Brown, Bill Wilson, Les Baer, Kimber, S&W, Dan Wesson, and Sig to name a few. I have three web sites, I am licenced in this city, state and have an FFL and do not see fit to use an alias because I have taken a great liking to Dave Sample. I am not ashamed to be Me, unlike some people here. Building 1911's is one of the least of the things I have done and I have had a lot of fun doing it my way. I do not build guns for jerks. I do not have a Students who are jerks. If you do not like what and who I am , that is your problem. Do not confuse me with someone who cares what other people think. Unless you are going to pay my bills, then I may give some thought to that. I am retired from 1911 work for the public and no one on any forum has enough money to get me to do work for them. I just take care of a few friends here and there. Stand at ease, Sir, and smoke 'em if you got 'em ! End Of Rant.

http://pic11.picturetrail.com/VOL368/953404/2976451/82693461.jpg

From this.............................

http://pic11.picturetrail.com/VOL368/953404/2976451/82693494.jpg

http://pic11.picturetrail.com/VOL368/953404/2976451/82693515.jpg

In about 15 Minutes. This is a PATRIOT 1911 Online Class COP back from Gunsite and put back together. This was built by a first time builder over the Internet. This is what I do now. I share what I know with others. I did what all the "Experts" said "couln't be done". Perhaps that is cause for the POOP. Who Knows? Maybe De Shadow Knows..........................

R.H. Lee
January 25, 2005, 10:38 PM
I think you've got the wrong forum, Dave. This is a thread about headspace, not a psychiatrist's couch. Sheesh. :rolleyes:

N.M. Edmands
January 25, 2005, 11:39 PM
:cool: Dave, I have a lot of respect for you and your abilities.I know of you and your reputation from my time in your area. [Congress, Az. ] My post was not intended as a personal attack. My appoligies if you thought it was.

Jim K
January 26, 2005, 11:18 AM
Hi, Tuner and guys,

The article on headspace was originally written for an M1/M14 forum, so it is somewhat slanted toward rifle headspace, but the principles are the same for a pistol.

Wear and peening of the locking lugs (barrel and slide) of a 1911 type pistol will take place regardless of linkdown time. Normal wear and compression occur in bolt action rifles, which have no automatic unlocking; the pressure involved in normal firing begins the process of lug and seat compression from the very first round. In a 1911 type pistol, locking is controlled by the barrel foot and unlocking is controlled by the link. Incorrect fitting in this area can result in only partial locking or a too early linkdown, which exacerbate the problem since the lugs are not fully engaged at the time of high pressure and only a part of the lug surface area takes the full strain. The result is excessive battering.

The same thing can happen if the barrel and/or the slide is soft. Designers usually specify that the barrel should be softer than the slide so it, as the cheaper part, will go bad first. (Designers recognize that wear and pounding will occur; the better ones try to make sure that most wear will occur on the cheapest and most easily replaced part.)

Jim

Dave Sample
January 26, 2005, 01:39 PM
I was asked a direct question, and I answered it. If you don't like the answer, then maybe you are on the wrong forum. I can duke it ourt with you guys with half of my mind tied behind my back. No Flame Taken, and please be aware that I am not one of the "Chronically Offended." I just wasn't put on this earth to take crap. Thanks for your nice words, Mr. Edmans. I have passed through your lovely little town many times and love it when the flea markets are in bloom!

1911Tuner
January 26, 2005, 01:57 PM
Jim said:

Wear and peening of the locking lugs (barrel and slide) of a 1911 type pistol will take place regardless of linkdown time.
********************

Exactly so, Jim. The example mentioned had the locking lugs set back about .020 inch, and more than likely created the excessive headspace condition. The battering and setback were due to a linkdown timing problem with the gun, as observed by the peening on the rear of the lower lug, causing the barrel to remain partially locked to the slide. Since the link had been replaced, I couldn't determine whether it had played a role in the problem, or if it was simply a matter of the frame's stop surface being a little too far forward. The barrel timed correctly when I installed it into three different guns here, but the barrel was a goner.

The point was/is that the chamber depth and dimension from stop shoulder to breechface aren't the only determining factors in headspacing in a 1911 pistol, and headspace will increase with use as the lugs compress. The lug battering and setback simply put the process into fast-forward.

I made a field gauge set of sorts that measures .910 (GO)and a second one that measures .915 inch(NO GO) for the purpose of determining the life remaining in a given pistol. When the pistol will close on the first but not on the second, I keep using the pistol until it will close on the .915 inch NO GO. At that point, I make a decision to either put the gun into semi-retirement for limited use, or rebarrel it. It's a system that I started using due to my addiction to shooting many thousands of rounds a year. New pistols that will close on the .910 GO gauge are normally tweaked for reliability and carried.
Works for me anyway... :cool:

Good thread, so far. Cheers all!

Toonah

Dave Sample
January 26, 2005, 02:05 PM
I relaced the link and pin. The barrel was fine for the gun it came in and I advised the owner how to make it run. He did not do this. Installing a Chinese barrel in three other guns proved nothing excet to re-afirm my belief that some people have too much time on their hands as seen by my posting on forums.
I could have made that Noinco run like a top.

1911Tuner
January 26, 2005, 02:18 PM
:rolleyes:

N.M. Edmands
January 26, 2005, 02:28 PM
:confused: :banghead: :confused:

Wichaka
January 27, 2005, 12:18 AM
So Tuner.........come here often?

What's that? I hear something.........it sounds like something self inflating again....... :scrutiny:

BluesBear
January 31, 2005, 05:13 AM
This is what I do now. I share what I know with others. I did what all the "Experts" said "couln't be done". And all for the low low price of $1200 plus another $1200 for the parts. :barf:

mister2
January 31, 2005, 12:25 PM
Back to the original Q

I understand how to headspace a bolt action, but how exactly do you headspace a semi-auto?

Differences in design call for different tools and different methods, even though headspace guages are always the same and herein (if I understand correctly), lies your original question. Which semi-auto rifle were you thinking of?

If FAL, go to

http://www.falfiles.com/forums/forumdisplay.php?forumid=19

....and if the archives aren't enough, there are members who are only too willing to help online or in person, if you're close enough.

Cheers,
MR2

Preacherman
January 31, 2005, 09:22 PM
Dave Sample, I don't have an axe to grind in this argument, in that I'm not a gunsmith (nor do I play one on TV, nor did I stay at a Holiday Inn Express last night... :rolleyes: ), and I'm not a major fan of the 1911 platform.

However, I have to say that in this and other threads, you're crossing the line of what is acceptable behavior on THR. I think that 1911Tuner has been very forbearing in allowing you to say some of the things you've said: if you had said them on one of the forums for which I'm responsible, you would no longer be posting. I say this simply to give you a "heads-up" that your language, attitude and approach need to improve BIG-time, because you're angering and alienating many users of THR (who are vocal in talking to the Moderators about you), and because we try to encourage an attitude of mutual respect among our members. We may disagree with them, but we try not to "diss" them or be rude in our dealings with them. If there are some with whom we can't communicate without such actions, they cease to be members.

Please give this some thought.

Art Eatman
February 1, 2005, 01:59 PM
Hmmmmph. I've been shooting 1911s for almost 60 years, and messin' around with their innards for over 25. All that means is that I'm old.

:D:D:D,

Art

Dave Sample
February 1, 2005, 11:55 PM
Thanks for the input.

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