Just so I understand "headspacing"

[jproaster]

Ok. As a new reloader (hope to start this week), I am still a little befuddled by "headspacing." It's one of those, "I get it", but read a little more from another source and "oh, maybe that's what it REALLY means."

So, I've spent the last few days reading from Lymans 49, Speers #14 and online threads here and there. The manuals have their various strengths, but I still lacked the true understanding- "is it the distance from the head to the case mouth" or "the space between the bolt face to the shoulder or mouth of a case?"

So I found this definition from wikipedia:

"In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt."
http://en.wikipedia.org/wiki/Headspace_(firearms)

Therefore I've come to understand that one of the following, based on case type, is the forward end of headspace (shoulder, case mouth or forward side of a rimmed head [like 30-30]) and and the bolt face when the action is closed is the rear end.

So then is "headspace" the distance between these two points from the sentence above?

And the purpose of proper "headspace" is to prevent the case from moving during firing? Especially since movement will allow gases to potentially escape rearward into the breech.

Caveat, I'm still learning about guns too; I've been shooting them for about 3 years. But recently have decided to become dedicated to really knowing them.

Thanks for not chewing me out for not reading more before posting. If I write it out, then I'll know what I know.

john

[mrawesome22]

A bottle necked case headspaces off the shoulder. When the shoulder of the case touches the shoulder of the chamber, this is as far as the shell can go foward in the chamber. The distance between the case head and bolt face is the headspace.

A semi auto, straight walled, case headspaces off the case rim. When the case is as far forward in the chamber it can go, the distance between the case head and the bolt face is the headspace.

A rimmed straight wall revolver round headspaces off the rim. When the rim is touching the cylinder, the distance between the case head and the back of the frame is the headspace.

[CS86]

I am still a little befuddled by "headspacing." It's one of those, "I get it", but read a little more from another source and "oh, maybe that's what it REALLY means."

I've kind of been the same way. I feel I understand head space, as far as bottle neck cases, and how far off the chamber shoulder should be.

But what happens if you don't bump the shoulder back enough and you are slightly in contact with the chamber? I had problems with cases before that I had to keep adjusting my die down because I was having to force my bolt down due to not bumping my shoulder back far enough. Then I've read that you should have the case .001-.003 off the chamber shoulder. If you don't have the tools how do you know that you have provided enough headspace and what happens if you are in contact with the the chamber?

[mrawesome22]

If you have no headspace, like you already figured out, they will be hard to chamber.

The bump of .001" to .003" you referred to , ensures the bolt closes easily, and case stretch is kept to a minimum.

When you push the shoulder back too far, the web gets stretched. The web is where the thick metal of the case head meets the thin metal of the case wall.

Think of bending a paper clip. If the bend it at a 90 degree angle over and over, it will break very soon. If you bend it at only a 5 degree angle over and over, it will go much, much longer before breaking.

Same with the case head. The less distance it is stretched, the longer it will last.

[F. Guffey]

Case travel, I am the fan of cutting down on all that case travel. then there are ‘bumpers’ they bump the shoulder back .002 thousands, with out mentioning one word on how they accomplish such a feat, me? I am the supporter of the tool ‘the feeler gage’ the companion tool to the press, the feeler gage, the verifier, the transfer, the standard, the feeler gage, and I am the fan of knowing the difference in length between the case from the head of the case to it’s shoulder and the length of the chamber from the bolt face to the shoulder of the chamber.

The difference in length? as opposed to chambering a round and pulling the trigger.

F. Guffey

[er775]

I understand exactly what you are saying, when I was starting to get my set up all together and studying up on everything, no manual out there would give a straight foward answer about headspace, I have the Hornady 7th edition and reading about headspace in that manual I was lookin like a monkey doing math

I have talked to enuff people and and asked enuff questions that I understand it better now and I agree with your last comment about not getting chewed out for asking a question like this, there are folks on here that store alot of knowledge in their heads and are more than happy to share it without making you feel like a moron and then there are the ones that are just plain a--holes that would be better off keeping their info to themselves cause most of the time they talk out of their poop shute and have nothing of value to add to the conversation ( if you read that and it offended you, then I`m probably talkin about you , sorry get over yourself )

Thanks for asking the question cause there are alot of us out here that are wanting to know the same info, and thanks to all of you out there that respond with your knowledge in a respectful and courteous manner it is much appreciated

[Bart B.]

In wearing out several 30 caliber barrels for both rimless bottleneck and belted bottleneck cases reloading thousands of rounds of ammo for them making all sorts of before and after case measurements, I'm convinced some popular beliefs about cartridge case behavior when fired is not well understood by folks.

First off, when the case is chambered in a rifle whose bolt has an in-line spring-loaded ejector, that ejector pushes the case as far into the chamber as its headspace allows. For rimless bottleneck cases, the case shoulder stops against the chamber shoulder and there's usually a small gap or clearance between the bolt face and the case head. For belted bottleneck cases, some will stop as their belt is pushed against the belt ridge in the chamber and there'll be a small gap between the bolt face and case head as well as between the case shoulder and chamber shoulder. In rifles with ejectors external to the bolt (Mauser styles that slide in a slot on the bolt), the case is a bit loose lengthwise in the chamber and will rest at various places depending on where the extractor pushes on it.

Second, when the firing pin strikes the primer some cases stay where they are in the chamber but others will move further forward. A belted bottleneck case with its belt held against the headsapce ridge in the chamber will stay there, the round will fire and the case shoulder gets blown forward while the case body grabs the chamber walls then the case head gets pushed back against the bolt face. If a belted bottleneck case has its shoulder against the chamber shoulder, the case shoulder may be set back a thousandth or so by firing pin impact increasing the space between the bolt face and case head, then the round fires and pressure expands the case body hard against the chamber walls while the case head gets pushed back against the bolt face. For rimless bottleneck cases, the firing pin drives then hard against the chamber shoulder and that sets the case shoulder back, the round fires and pressure expands the case body hard against the chamber walls while the case head gets pushed back against the bolt face. In virutally all instances, the case length from case head to case mouth is shorter after firing than before 'cause the case expands in diameter when its fired and that draws brass out of the chamber neck. Do your own before and after measurements with the right tools and you'll see this happen.

After a case is resized by a die, it gets longer from head to case mouth 'cause you've made its body and neck diameters smaller and that cartridge brass has to go somewhere. With the right type of sizing die and correct tools to measure what's happening, you can resize cases just enough to enable proper chambering for accuracy, safety and long case life.

The headspacing parts of case and chamber are established for safety. It's best if only a few thousandths inch difference between the case and chamber dimensions exist. Otherwise, accuracy will go down hill and the case may stretch too much causing metal fatigue such that the case cracks or splits letting hot, high pressure gas escape onto your body parts. Done correctly, managing head space is safer than crossing a street in traffic when you've got the walk signal.

[CS86]

I would incrementally adjust the FL die untill the bolt closed smoothly and I felt like I had headspace. Can a person feel if there is a .001 interferance. What if the interferance is so slight that the shoulders touch? Does is pose a problem? Possibly accuracy or pressure issues? Or is it acceptable to have a slight interferance as long as your bolt shuts ok.

[old roper]

Here is case and chamber SAAMI and go to chamber and you see where headspace is this is 30-30

http://www.saami.org/PubResources/CC...Winchester.pdf

Here is belted mag
http://www.saami.org/PubResources/CC...n%20Magnum.pdf

Here is 30-06
http://www.saami.org/PubResources/CC...pringfield.pdf

[mrawesome22]

That is what neck size dies do. They don't push the shoulder back at all which leaves the brass in a fireformed state.

The problem is that nothing man made is perfectly round or perfectly concentric.

So after a few firings, there is so little tolerance between the case and chamber that the cases will become hard to chamber.

I've switched over to FL sizing every bottle neck I load. When set up properly, I enjoy easy chambering every time, accuracy is just as good as well as case life compared to neck sizing.

[jproaster]

Thanks for the replies guys. I really enjoy knowing what is happening so that I can understand why reloading details are what they are.

BTW, why is the mouth, neck and shoulders at the top and the head at the bottom?

john

[Bart B.]

cs86 asks:

Or is it acceptable to have a slight interferance as long as your bolt shuts ok?

I don't think so. Especially with all factory rifle actions and barrels and most of the custom ones if accuracy is important to the shooter. As there's a bit of clearance between the bolt and receiver, a small amount of slop's there. If the bolt closes on a loaded round exactly the same for each shot, the very best accuracy will happen.

When the bolt binds against a loaded round, the bolt face moves around on the case head. As all factory bolts' face ain't perfectly square with the chamber axis, when the bolt binds on case heads the locking lugs seat at different places. Firing a new case in such situations causes that case head to flatten out against that unsquare bolt face. Resizing that case to reload it doesn't square up its case head. When it's chambered and its' high point aligns with the bolt face high point and the fired case shoulder wasn't set back far enough, binding happens and the locking lugs seat all over the place instead of only one place. And that causes the barreled action to whip differently from the inconsistant stresses set up in the barrel. Bullets leave at slightly different angles relative to the line of sight. Proper tests have shown from 1/4 to 1/2 MOA increase in test group size from such binding.

However, if you and your rifle don't shoot that well, then binding up the bolt in your rifle on case heads may not matter at all.

Top competitive and other accuracy-obsessed shooters reloading bottleneck cases with a full length sizing die use a case gage, such as the RCBS Precision MIC, to measure the case head to shoulder "space" or length on a fired case. Then full length size the case and measure the case again. Best practice is to set the fired case shoulder back about 2/1000ths inch. Just adjust the die up or down in the press until the desired shoulder setback is attained. That gives record-setting accuracy, long case life and all sorts of other good stuff. This is what Sierra Bullets does testing their products for accuracy; very few people shoot their bullets as accurate as they do.

[Bart B.]

F. Guffey comments:

then there are ‘bumpers’ they bump the shoulder back .002 thousands, with out mentioning one word on how they accomplish such a feat

Here's how its usually done. I and others have been doing this for almost 50 years; it's nothing new. Back then, we had to make our own tools as nothing long ago was neat as what's available now.

First, the fired bottleneck case has its case measured for case headspace from head to shoulder reference with a tool such as the RCBS Precision MIC. It's thimble's removed, a fired case is inserted base down then the thimble's screwed on gently until it's gaging surface stops against the fired case shoulder's reference point. The thimble's read off the index line same way as a micrometer is as it's graduated the same way. That number represents what the fired case headspace is.

Second, the fired case is full length sized in a die for it then measured again the same way. The reading after sizing is compared to the case reading with the tool before it was full length sized. It shows if the angled case between the neck and body (hereafter called the shoulder) has moved relative to the case head.

Third, if the reloader intended to "bump" the shoulder on the fired case back 2 thousandths after full length sizing it, then one of three things now exist. Two of those things require readjusting the die height in the press.

If the sized case headspace is now 2 thousandths shorter than before, it's been "bumped" back the desired amount.

If the sized case headspace is now more than 2 thousandths shorter than before, the die has to be readusted up a tiny bit.

If the sized case headspace is now less than 2 thousandths shorter than before, the die has to be readusted down a tiny bit.

Loosening the die's lock ring then rotating it about 1/10th inch in circumference on the die will relocate it about 1 thousandth of an inch. Should the die need to be moved down 5 thousandths so it will bump the fired case shoulder 2 thousandths back, then turn the lock ring about 1/2 inch in circumference on the die such that it moves higher up the die. Then lock the ring in place and screw the die down into the press.

Compare the above to thse instructions from RCBS for setting up a full length sizing die to bump the fired case shoulder back 2 thousandths.

http://www.rcbs.com/downloads/instru...ecisionMic.pdf

[dmazur]

The distance between the case head and bolt face is the headspace.

Actually, the Wikipedia link is the correct definition. Which makes the above statement misleading.

The problem is, the term "headspace" has been used to refer to at least three different dimensions (in various books and articles I've read.)

1. The distance from the bolt face to the surface that stops forward motion of the cartridge.
2. The distance from the head of the cartridge to the datum point on the shoulder (or front surface of the rim, or case mouth, depending on design)
3. The difference between the above two dimensions.

IMO, it would sure simplify things if we referred to the first as "chamber headspace", the second as "cartridge headspace" and the third as "headspace clearance".

Unfortunately, the various uses of the word headspace seem to be embedded in the vernacular and refuse to be rooted out...

Considering this, I can only advise that the new reloader read carefully and try to determine what the context is. Sometimes, the dimension itself is an aid to determining what is being described.

By that I mean that if the dimension is 2.049" or so, the distance to the shoulder datum is being described. But if the dimension is 0.006" or some other small number, the difference between the cartridge and the chamber is being described.

Something else that is perhaps important to understand is the difference between true excess headspace (caused by bolt lug wear, for example) and what is sometimes termed "induced excess headspace" (caused by over-resizing.) As far as the cartridge is concerned, both conditions are identical and can cause case head separation if the excess headspace is large enough.

If you have true excess headspace, you can't fix it by adjusting the resizing die. You have to make the headspace shorter in the rifle, by installing a new barrel or possibly a longer bolt (if available in sizes.) The die isn't the solution.

If you have induced excess headspace, you just adjust the die for less resizing.

[dmazur]

And the purpose of proper "headspace" is to prevent the case from moving during firing?

I thought I'd answer this one separately, as it isn't related to the definition of headspace.

AFAIK, proper cartridge headspace is necessary to prevent case head separation due to stretching of the case near the case head. (Caused by the case walls gripping the chamber at the time of firing, but the heavier case wall near the head can't expand as much and so it moves back until stopped by the bolt face.)

It can affect primer ignition as well, if the case is not held tightly against the bolt face by an extractor. If the case head can't be reached by the firing pin, the cartridge may not fire.

As mentioned by others, it can affect chambering. Insufficient chamber headspace can make it difficult or impossible to chamber factory ammo. Induced insufficient headspace (caused by not resizing enough), can cause the same problem with a rifle with proper chamber headspace.

I don't believe it has anything to do with providing a gas seal. The case expands radially outward against the chamber walls to provide the gas seal, regardless of its position in the chamber.

[F. Guffey]

On another forum I have been asked not to type slower, or was I told not to say I can not type any slower, in any case, if I could type slower I would. I determine the length of the chamber first, I do not find it necessary to fire a case to determine the length of the chamber. I know it must be boring for the members of the ‘CENTURY CLUB –50’ AKA ‘HALF CENTURY CLUB’ to hear about my M1917 Eddystone with .016 thousandths head space, meaning the chamber is .016 thousandths longer from the face of the bolt to the shoulder of the chamber than the (minimum length case .000”) case from the head of the case to its shoulder. The first time I fired that rifle I fired it with .002 thousandths head space. Then there is ‘how to determine the length of the chamber, me? I determine the length of the chamber 2 different ways without a head space gage, remember, there is no such thing as a + .016 thousandths head space gage over minimum length.

Back to bump, in the real world when someone says “I bump my shoulders back etc., etc..” I ask, “How do you do that?” anyhow, I can’t, by the time the shoulder of the sizer die gets to the shoulder of the case the case body is in the grips of the sizer die body, once the shoulder of the case contacts the shoulder of the sizer die the case has few options, the case can compress, shorten, or lengthen and or both, and of course I control sizing by adjusting the die to and or off the shell holder with a gap determined with the use of a feeler gage, back to determining the length of the chamber first, knowing the length of the chamber allows me to adjust the sizer die for sizing.

F. Guffey

[Bart B.]

mrawesome22 states:

The distance between the case head and bolt face is the headspace.

While that's a popular belief, the firearms manufacturing, gunsmithing and reloading tool industry standard definition for that distance (or dimension) is "head clearance." But I well understand why folks use "headspace" to refer to that space between the bolt face and case head. It's just that dimensional specifications for headspace gages state the distance between the bolt face and the reference point in the chamber that stops the case from going too far into the chamber.

Head clearance for a given chamber is determined by subtracting case headspace, the distance between the case head and the industry standard point on the case used as a reference (shoulder on rimless bottleneck cases, front of the rim on rimmed cases, front of the belt on belted cases and case mouth on rimless straight cases), from chamber headspace.

One example, a .308 Winchester case with 1.627" case headspace (head to shoulder reference) in a SAAMI spec minimum chamber with a chamber headspace of 1.630" will have a head clearance of .003" when the round fires.

Another example, a .300 Win. Mag. case with .217" case headspace (head to front of belt) in a SAAMI spec minimum chamber with a chamber headspace of .220" will have a head clearance of .003" when fired.

A historical example, a .30-30 Win. case with .060" case headspace (head to front of rim) in a SAAMI spec minimum chamber with a chamber headspace of .063" will have a head clearance of .003" when fired.

A military example, a .30 Carbine case with 1.287" case headspace (head to front of mouth) in a mil spec minimum chamber with a chamber headspace of 1.290" will have a head clearance of .003" when fired.

The above aside, folks choosing to use different meanings for "headspace" may do so, but it makes understanding difficult by others who choose to use the languange of industry standards.

[dmazur]

Bart B. -

Thanks for the clarification.

I saw on the SAAMI drawing that only one dimension was marked as "headspace", and that was the chamber.

So "chamber headspace" is redundant. The correct term is "headspace". However, "chamber headspace" does make it unambiguous when discussing multiple dimensions.

What I called "cartridge headspace" is "case headspace" and "headspace clearance" is "head clearance".

I will try to use the correct terms in the future.

I believe I was correct in stating that "headspace" is often used to describe any one of the above three, and sometimes it seems to depend on the writer's whim...

[Bart B.]

dmazur, your comment:

It can affect primer ignition as well, if the case is not held tightly against the bolt face by an extractor. If the case head can't be reached by the firing pin, the cartridge may not fire.

This is another popular belief. And again, 'tain't what happens. Here's why.

All firearms' extractors lip distance from the bolt face is greater than the case rim's thickness; by a few to several thousandths of an inch and none of them pull the case head back against the bolt face (at least as far as I know).

Firing pins protrude a few dozen thousandths of an inch from the bolt face when in their full-forward position. Typical rifle pin protrusion is .055" to .060". More than enough to make up for a huge amount of head clearance or space between the bolt face and case head where the primer's virtually flush with.

When the firing pin strikes the primer, the case moves as far forward in the chamber that its "head clearance" allows and then stops, the primer doesn't fire until the cup gets dimpled next by the firing pin and crushes the priming pellet such that it explodes/detonates/burns-fast (whatever you prefer to use).

A good way to see this happening is use several reloaded rimless bottleneck cases each one having 1% less powder than the first one with a maximum powder charge. Fire those reloads in order, max load first down to the last one that's loaded with a powder charge 10% to 15% below max. Measure each fired case headspace with a gage in order. At about 5% below max, you'll notice that fired case headspace starts getting shorter and primers are backed out a bit. I've done this with a .30-06 case and at 10% to 12% below max, fired case headspace is .010" less than where it started at and the primer's backed out about .010". To me, that's proof that the firing pin pushes a rimless bottleneck case hard enough into the chamber that its shoulder gets set back before the round fires. And with loads reduced enough, there's not enough peak pressure to expand the case fully back until its head stops against the bolt face.

Note that for rifles with a spring-loaded plunger, in-line ejector in the bolt face, that pushes the case forward until it stops against the chamber headspace point; there's a gap or space between the bolt face and case head. Extractors have enough clearance to let this happen. Besides, there has to be enough clearance between the bolt face and extractor lip to let the extractor slide easily over the case rim's lip when chambering the round.

[Bart B.]

dmazur comments:

If you have true excess headspace, you can't fix it by adjusting the resizing die. You have to make the headspace shorter in the rifle, by installing a new barrel or possibly a longer bolt (if available in sizes.) The die isn't the solution.

That's true, usually. The one exception I came across was a Win. 70 that had a used factory .30-06 barrel installed to replace the original factory .270 Win barrel. And some folks end up with "true excess headspace" for one of several reasons.

I fixed mine by setting a .30-06 full length sizing die in the press such that fired .30-06 cases from another rifle would full length resize without setting the shoulder back. Instead, the die was set up much like folks do when they want to "partially size" the fired case neck and not touch the fired case shoulder at all with the die. This squeezed the body diameters down enough to move the shoulder forward about .009" to have about .005" head clearance in the barrel's chamber. It's actual chamber headspace was 2.065" (SAAMI reference point of .375" diameter on the shoulder, about .050" back from the neck) and the fired cases averaged 2.051" before resizing that put them at about 2.060". Firing those cases in the replaced barrel ended up with about 2.063 case headspace.

These numbers are what I remember from doing this decades ago, but they're within a thousandth of reality. Point is, one can correct excessive headspace if it's not too excessive.

[jproaster]

So, if built my own ar15, do i need to obtain a gauge to determine chamber headspace?

And (forgive me if I've just missed it above), what is the purpose of head clearance? "Why isnt' the bolt face smack on the head?" may be a blunter way of asking.

john

[dmazur]

Good thread (I think).

I've read of (but not experienced) .45ACP that fires based on the extractor, and with excess headspace. Reliability is questionable. The example was .45ACP fired in a .460 Rowland chamber.

I can see that, without the shoulder to provide a positive stop, and with this huge amount of headspace (essentially in a barrel chambered for a longer case), the extractor may just not be up to the task.

So the ignition reliability myth is just that, and the main concern is enough headspace for proper chambering (especially in gas guns), but not enough to shorten case life or risk case head separation.

As to the question about "head clearance", I believe that is another expression for proper headspace. If you have zero head clearance, you may be just fine for a bolt-action rifle (which has tremendous camming power when chambering) but not so good for a semi-auto rifle (which does not.)

In other words, a certain amount of head clearance is necessary for ease of chambering. Depending on the action, this can be critical. (Look up "slamfire".)

I haven't built an AR-15, but I understand they are essentially drop-in if all the parts are mil-spec. Here's a link -

http://www.ar15.com/mobile/topic.html?b=3&f=4&t=315921

[Bart B.]

jproaster asks:

So, if built my own ar15, do i need to obtain a gauge to determine chamber headspace?

Yes. But you'll need two gauges; a "GO" and a "NO-GO." Their first job is to ensure the bolt fit and chamber dimensions are good so chamber headspace is within tolerances. It's best if chamber headspace is at the minimum end of its tolerable range.

Do you know how to use headspace gages to ensure the chamber is OK after the barrel's fit to the receiver?

After the rifle's built, you don't need a headspace gage any more. Fired case headspace can be measured well enough with a gage made for that, such as the RCBS Precision MIC.

[Bart B.]

dmazur mentions:

As to the question about "head clearance", I believe that is another expression for proper headspace. If you have zero head clearance, you may be just fine for a bolt-action rifle (which has tremendous camming power when chambering) but not so good for a semi-auto rifle (which does not.)

"Head clearance" is not the same as "proper head space." One's a very small measurement typically only a few thousandths of an inch. The other's much greater; up to almost 3 inches. And both are in different areas inside the chamber.

You'll never be fine if you have to force the bolt closed on a round in a rifle. That's a sure way to hurt accuracy unless the bolt has virtual zero clearance to the receiver and the bolt face is perfectly square with the chamber. Very few benchrest rifles are built that tight. Like Sierra Bullets' first balistic tech stated after trying all sorts of fired case sizing popular in the early '50s to see what worked best for testing their products for accuracy. He learned some clearance between the case and chamber's important for accuracy. His favorite words to that were "The case has to fit the chamber like a [color=#FF0000]█[/color][color=#FF0000]█[/color][color=#FF0000]█[/color][color=#FF0000]█[/color] in a punch bowl." There's a few of us around that have heard that straight from his mouth.

[Slamfire]

How you want to call it, cartridge headspace is critical for function and safety.

Cartridge cases cannot expand forever without rupturing somewhere on the side wall. That is why the difference between Go and No Go is typically 006”. Cartridge stretch above that and there is a real risk of case head separation, depending on brass hardness.

Lee Enfields stretch so much that cases fired in those things develop case head separations in a couple of reloads.

You don’t want an interference fit because when the barrel and lugs relax, they will be in yield during combustion, the interference fit in the chamber will be even more, causing hard extraction or a failure to extract.

This is why hot loads cause sticky extraction. Pressures expand the case, barrel and lugs, and once the pressure is removed, there is an interference fit.

Excessive headspace can cause lug/receiver seat peening. I understand this is more of an issue with machine guns, they fire a lot more rounds, but still, the further the case head is from the bolt face, the faster it will be moving when it impacts the bolt face.