Help me understand "Head Space"

[Prof Young]

Loaders:
I think I don't understand what "Head space" is.
Help.
Live well, be safe
Prof Young

[Unclenick]

Headspace is just how much front-to-back room there is for a loaded cartridge inside a closed chamber. It is determined by measuring from the breech end of the closed chamber to whatever surface the front of the cartridge seats against when it is pushed in all the way. That's all there is to it.

What is confusing is the term "head" being part of "headspace". It's an historical artifact and is not literal in most cartridge designs now, except in rimfire cartridges. In a rimfire cartridge the rim is the pressure head (the barrier to pressurized gas escaping), so the front-to-back space the rim fits into in the closed chamber is literally the space for the pressure head, or headspace. The term simply got carried forward after cartridges with solid rims began to appear, and then was carried forward again when cartridges that didn't stop on their rims began to be designed in the late 19th century. At that point, in a relatively short time, we went from cartridges designed to stop against the rim to cartridges that stop against case shoulders, case mouths, or case belts, none of which are part of the head. But the distance from the breech to that stopping surface continued to be called headspace anyway, just out of habit or tradition.

Incidentally, the SAAMI site has a Glossary on their Info and Specs page (third item from bottom) that can be useful to sorting out terminology. In addition to "Headspace", look up "Head Clearance", which some mistake for headspace.

[F. Guffey]

Head space in the form of a question: What is the length of the chamber from the shoulder/datum to the bolt face?

Then there is the question: What is the length of the chamber in thousandth.

Then there is the question: What should the length be? and, What can it be?

Then there is the question: How, 'is it possible' to measure the length of the chamber from the shoulder/datum to the bolt face.

And then? There is the matter of determining the difference in length between the case when measured from the shoulder to the case head when compared to the length of the chamber from the shoulder/datum to the bolt face?

Clearance, the difference is clearance, we have reloaders that have settled on .002". Review the responses, "I bump the shoulder back .002" ? And then? there is my favorite response in the form of a question: Pond, James Pond ask "HOW!? When the question comes up reloaders are conditioned to respond with "Bump the shoulder back .002", Pond, James Pond wants to know 'HOW!?'

Then there are those that declare the case has head space and every tool is a head space tool. I have comparators, transfers, standards and tools that verify, My head space gages are head space gages. I make tools that measure the length of the chamber from the shoulder to the bolt face, these tools aid me in determine the length of the chamber in thousandths, the tools are not head space gages.

F. Guffey

[603Country]

Envision a series of short port-a-potties of varying heights, and all having a hole cut for your head to go through. When you enter one, stand on the floor (bolt face) and stand with your head through the hole, then there will be a distance from the top of your shoulders (case shoulders) to the roof of the port-a-pottie (the end of or the shoulders of the chamber). That would be the headspace. Too little would cause you to force your shoulders against the roof (an overly snug fit of round to chamber). The right fit of you to the port-a-pottie, for this analogy at least, would allow you to stand in the p-a-p freely, with a small amount of gap between the roof and your shoulders.

Of course, to keep this going, every time you use the p-a-p you would increase in height a small amount till your shoulders were tight against the roof. At that point we'd have to run you through a body die to bump your shoulders back a touch to re establish the correct fit to chamber.

[Jim243]

I really got a kick out of 603's comment (LOL). But, my understanding is he is talking about the wrong end of the cartridge. The "HEAD" is the bottom of the cartridge not the top. So it would be how tight you are sitting on P-O-P and how far your shoulders would be to the hole in the top and how much you are being pushed down to the seat. (bolt face).

Or that at least that's how I understand it. Too much "Head space" and you will not fit in the P-O-P, too little "Head space" and you will need a lot of toilet paper, just the right amount of "Head space", Priceless. (very comfortable.)

Jim

[Pond, James Pond]

And then? there is my favorite response in the form of a question: Pond, James Pond ask "HOW!? When the question comes up reloaders are conditioned to respond with "Bump the shoulder back .002", Pond, James Pond wants to know 'HOW!?'

Here's a response in the form of a statement: glad you like it.

To the OP, you can have a look at this thread and this one. I started these when I was as confused as you. Rest assured, it is pretty straight forward.

All that is involved in adjusting your cases to suit the dimensions of your chamber is adjusting your full-size die so that the distance from a given point on the slanted part of your bottleneck case to the base of it is "X" inches.

X is just how ever many thousandths of an inch you want your case to be, in comparison with how long it was, between those two same points, after you'd fired it in you rifle.

I work in mm, but as an example, my .223 cases, once shot in my rifle were predominantly 37.62mm from the base to the slanted part (shoulder). I wanted to set those shoulders back by 0.075mm, or 0.003".

So I wound the F-L die in as recommended in the instructions, sized that case and measured it again, from base to shoulder and found it was 0.1mm or 0.004".

So I wound it out a fraction, took another case with the same length of 37.62mm and tried again. As it happens I hit my target of 37.55 that second attempt.

The trick is finding something that you can use to rest on the shoulder, but clear the top of the case, allowing you to use calipers to measure. There are gauges, but I've used a M8 metric nut to measure mine.

[GeauxTide]

One other thing is "how" the different cartridges headspace is measured, i.e. Magnum rifles on the belt, standard cartridges on the shoulder, pistol cartridges on the mouth.

[Nlight1]

The port-a-potty analogy is brilliant.

[Prof Young]

Loaders:

Okay, I think I get the idea.

So when you are using a shouldered cartridge like say my 223 the shoulders (not the end of the necked part) are what stop the forward movement of the cartridge as I push it into the chamber.

On a rimmed cartridge like my 45colt the rim stops the forward movement.

On a rimless cartridge like my 9mm does the business end of the brass stop the forward movement?

I can see how all this would have a serious effect on case length especially for the rimless.

So much to learn.

Live well, be safe
Prof Young

[HiBC]

If I may,Uncle Nick,try to clarify a fine point.
Unless I have it wrong:
The "headspace" is in the firearm.It is established when the chamber is cut in the barrel.It is important that the chamber is cut to the proper depth.
There are three gages.These gages are made to the SAAMI standards for the tolerances on a proper chamber.
The bolt should close on a "go" gage,and not close on the "no go".This is the standard for a new chamber.
The function of the "field gage" is to identify a firearm that shows enough wear it should be taken out of service.
As I said,all this is in the firearm.
But there is another player in the game,the ammunition.Generally we can expect factory ammo to be loaded to SAAMI specs,so it will properly fit a properly headspaced firearm.

The amount of clearance between the ammo and the headspacing feature(rim,shouder,belt,case mouth) is the head clearance.

The handloader can induce excessive head clearance to the ammunition.It is possible to set a shoulder back more than necessary or desirable.
While,at first glance,the head clearance for a belted or rimmed case is fixed,The handloader can fine tune the effective head clearance by limiting shoulder setback when sizing.

We need enough head clearance for the receiver to easily lock into full battery.
Beyond that,excessive head clearance shortens brass life.

When short ammunition results in excessive head clearance,the symptoms and problems are the same as a firearm with excessive headspace.

This will be true even though a gunsmith can check headspace on the firearm and the firearm will accept the "Go" gage and will not accept the "no go " gage,so the firearm headspace is perfect.

[mehavey]

Originally Posted by ProfYoung

So when you are using a shouldered cartridge, ...the shoulders ...are what stop the forward movement of the cartridge as I push it into the chamber.

On a rimmed cartridge ...the rim stops the forward movement.

On a rimless cartridge ...the business end [mouth] of the brass stop[s] the forward movement...

Aaaah... Errrrrr....
RIGHT !

[F. Guffey]

This will be true even though a gunsmith can check headspace on the firearm and the firearm will accept the "Go" gage and will not accept the "no go " gage, so the firearm headspace is perfect.

So the length of the chamber from the datum to the bolt face is .008" longer than a minimum length/full length case when measured from the datum/ shoulder to the case head if the chamber is .001" shorter than a no go-gage length chamber.

I would not call that perfect, given the opportunity to cut the chamber and I wanted 'perfect' according to SAAMI I would cut the chamber to go-gage length and use minimum length/full length sized cases. In the perfect world I would have .005" difference in length between the chamber and case when measured from the chamber shoulder to the bolt face and then compared to the case from the shoulder to the case head.

Perfect? I off set the length of the chamber with the length of the case. It is possible to measure the length of the chamber with one gage, problem? Convincing reloaders and smiths it can be done.

I will take a liberty with levity, only because someone thought the outhouse was funny.

Jimmy Dean ask the audience a question. He wanted to know why the chicken crossed the road. And of course no one knew.

So? He could not wait to tell them.

He said the chicken crossed the road just to show the opossum it could be done.

Go, no and beyond, my presses and dies have threads. I do not have go, no and beyond threads.

F. Guffey

[mikld]

If you are a new reloader, go back to Uncle Bick's first post. Take all the others with a grain of Bullseye...

[Metal god]

The port-a-potty analogy is brilliant.

I agree but I think Jim243 is right , the OP has it backwards . I would think you would need to stick you head in the toilet section of the port-a-potty and when your feet can touch the roof . That's your head space . The only real problem I see with this is getting all that blue coloring off your face when your done .

[44 AMP]

On a rimless cartridge ...the business end [mouth] of the brass stop[s] the forward movement...

Except, of course, when it doesn't...

The plan is for the rimless "straight" case to headspace on the case mouth, but the reality is sometimes different. It depends on the gun its fired in. Some guns will chamber and fire drastically shorter than standard length cases. These cases never touch the ledge in the barrel that correct length cases headspace on. But they still fire.

Because, in SOME guns, the over short case will be held against the breechface by the extractor, and so be within "reach" of the firing pin, and held there, so they fire.

But only in SOME guns.

Hope this helps a bit, despite adding to the confusion.

[mehavey]

Some guns will chamber and fire drastically shorter than standard length cases... [held by the extractor].

But that is the case with ANY cartridge that is too short -- including bottleneck rifle cases.
In the end, the design headspace stop for rimless/straightwall* cartridges is the case mouth.

(Which is not to say that gross blunders might not still "fire")
To wit: a 1911/45ACP Chamber (Rim: 0.480"/Case length: 0.898"
will fire (1X) a 40 S&W (Rim: 0.424"/Case length 0.850"
simply because the extractor held the (much) smaller cartridge against the firing pin.


*Yeah, I know: "There's a slight taper"

.

[F. Guffey]

Loaders:
I think I don't understand what "Head space" is.
Help

What it is? Headspace. Where it is? How to measure it? Then there is that very boring conversation that starts with "Hatcher said etc.". No one ever distinguishes the difference between receivers when discussing head space.

I have fired 8mm57 ammo in 8mm06 chambers I could say think about it, that does not work because I would be required to explain the part about 'what to think about'.

When an 8mm57 is fired in an 8mm06 chamber the 8mm shoulder is .121" + clearance from the shoulder of the chamber. then there is the 'auto response' as in forum members condition to respond to trigger words and phrases. It always starts with the firing pin strikes the primer and the whole thing (bullet, powder and case) take off and collide with the shoulder of the chamber. Like the song "Along cam John", and then, and then the primer is crushed.

and I continue to say 'think about it', the shoulder of the 8mm57 is .121" + from the shoulder of the chamber, the shoulder of the 8mm57 never makes it to the shoulder of the chamber.

If there was some way I could tie all of this in with an outhouse or a port-a-potty I would. Then there is the other one, firing a 308 W in a 30/06 chamber. The difference in length from the shoulder of the 308 W and the 30/06 is .338". 308W ammo has been fired in 30/06 chambers, with the shoulder of the 308W shoulder .388" away from the shoulder of the 30/06 chamber shoulder a shooter would think 'case head separation', when the case is ejected it comes out looking like a short 30/06 case with a very short neck. Again, the shoulder of the 308 W never makes it to the shoulder of the chamber'

I am the fan of cutting down on all that case travel.

Again, A friend build a wildcat, he had 5 case head separations out of the first 10 cases fired, when he told me about the problem I had to ask "What receiver were you using?" I could have fixed the problem before he left the shop, I could have fixed the problem after he got to the range, I could have fixed the problem by forming cases to fit the chamber before he started.

F. Guffey

[603Country]

Mr Guffey, no offense intended, but your first explanation of headspace was what caused me to try to find some way to simplify an explanation. The Port-a-Pottie analogy came to mind.

There's no doubt that you understand what you are saying, and I understand it too (about 75% of the time), and the depth of your knowledge is truly impressive, but I wasn't at all sure that the OP would make any sense of it. So...I made a pass at trying for a more easily understood explanation.

[mikld]

Well 'Country, 75% is pretty good. I usually can only get 40%-50% of what he posts...

[HiBC]

Mr Guffey,I try to see merit in what you have to say.I really do.

Here is what I seem to find.You believe you are the only one who carries the True Knowledge,and the rest of us are stumbling in the dark,dazed and confused.
From your very tall shop stool,you blather on about your own greatness,while dispensing almost zero useful information.
You mostly muddle and confuse issues that other folks are trying to learn about.
In other words,you offer little,but make things more difficult.

My suspicion...what I think maybe I read between the lines,you spent time as a hack gunbutcher with no regard for the use of headspace gages to chamber a rifle.

Then your answer to your poor work was to have your victims create workaround ammo to feed your creations.

I wrote to serve the OP's question.You wrote to be a critic,to muddy the water,and to serve your own ego.You offer nothing of benefit.

My suggestion to those who want to learn something,listen toUncle Nick,and others serious about your question,and consider Mr Guffy for whatever entertainment you may find there.

[B.L.E.]

At least with rimless bottleneck rifle cases, you can leave the case unresized and only resize the neck so it will grip the bullet. Since the case blew out to fit your chamber, you essentially have zero headspace from the second firing onward.

With rimmed cases, if you have excessive headspace, you are stuck with it. Some rimmed bottlenecks can headspace on the shoulder.

What is the consequence of too much head space? Short case life. I used to have a Ruger Super Blackhawk that ate up cases at a ridiculous rate unless I downloaded to .44 special plinking velocities. What happens it the case expands against the chamber walls gripping the chambers while the pressure pushes the case backwards against the breech. This movement stretches the case until the head separates. I would only get about 4 reloads out of brand new brass before this started happening, except with a batch of Norma cases which seemed to be immortal. I suspect they may have had thicker rims.

My current S&W Model 29 .44 Mag does not have this problem, cases last until the mouth splits.

[StripesDude]

Bravo HiBC. Well said.

[Gunplummer]

Hornady reloading manuals explain headspace pretty well, although when you start to dabble into foreign and even US military chamberings there will be variations in spec call outs. Even MILSPEC brass is different. The military 30.06 and the .308 chamber specs are called from a different point than SAMMI specs are. Most of you may have noticed that the more recent load books have more than one section for the .308 cartridge. I would suggest reading a good reloading book. I have great respect for P.O. Ackley's writings, but in his Vol. I Handbook for Shooters & Reloaders, his explanation of rimless headspace is just wrong. It is a difficult thing to explain even if you understand it, which I am sure he did.

[603Country]

Some of you guys are being too tough on Mr Guffey. For a while, a long while, I just flat didn't understand much of what he was saying, but I could see that there was much knowledge and experience there. So I'd take a couple of Advil and focus real hard on converting what he said to a language I could understand. I'm getting there slowly and I am understanding much more of what he says. He makes a lot of sense (after translation). He isn't an easy read, but that's Ok. I like his perspective. It is different.

He may well think that there are idiots among us. He may be right.

[B.L.E.]

When an 8mm57 is fired in an 8mm06 chamber the 8mm shoulder is .121" + clearance from the shoulder of the chamber. then there is the 'auto response' as in forum members condition to respond to trigger words and phrases. It always starts with the firing pin strikes the primer and the whole thing (bullet, powder and case) take off and collide with the shoulder of the chamber. Like the song "Along cam John", and then, and then the primer is crushed.

I'm pretty sure that the inertia of the cartridge is enough to allow the firing pin to crush the primer, especially with the speed of a typical bolt action firing pin.