For the sake of understanding,hammie,the flat face of the breech is one important surface.
If you set a piece of brass vertical on the table,the case head is resting on the table.(I'll agree the "head" includes more,but for our immediate purpose,"headspace" think of the base surface)
In the gun,we might say the case head rests against the breech face.
But I say that with reservations.We'll talk about it more in a minute.
For now,think about the case head resting against the breech. That is one boundary for the brass.
There are different systems to establish the forward boundary for the brass.
One is a rim,like the 30-30 or 22 rimfire.
Another is the case mouth,like 9mm or 45 ACP.
Another is a belt,like most magnums.300 win mag is an example
another is the shoulder of a rimless case.Specifically,SAAMI gives a diameter,a theoretical ring that rests on the shoulder normal to the axis of the chamber.
In each case SAAMI gives a dimension from the breech face to the appropriate headspacing feature. The minimum and maximum acceptable dimensions from the breech to the headspacing feature are the headspacing dimensions A gunsmith worthy of working on a gun will use a set of headspace gauges ,a "Go" and a "No Go" to confirm the chamber is cut to the proper depth. The headspacing feature is precisely ground into the chamber reamer.
The chamber is cut to acceptable standard of HEADSPACE when the rifle chamber accepts the "GO" gauge and it does not accept the "NO GO" gauge.
That's it. That's headspace.Its all about the breech and the chamber of the firearm. We don't even have any ammunition. Note that there is a tolerance range of acceptable chambers.(We will save "field gauge" for another day...well,OK.Old guns or abused guns can wear and develop a longer headspace.There is an acceptable amount of wear before the gun should be taken out of service.If a gun accepts the "Field gauge" its a reject)
OK,but we want to shoot ammunition in our gun.The ammunition has a dimensional relationship with the rifle chamber. We have to control that,too.
We trust factory ammo to be right. SAAMI spec. It usually is.
That would be all most folks need to know,except for those pesky reloaders.If new ammo was fired once in SAAMI spec guns,and thrown away,we would not need to worry about all this barn carpet.
But reloaders use brass over again.They want the bolt to close,even if the brass was fired in a long chambered gun and stretched a bit. It might go in a short chambered gun. We want to minimize case stretch to make the brass last longer.
But only the gun has headspace. How do we reloaders talk about this? How do we help newbies with questions? Its not easy because A) You need help with "case head" (That's OK)
So now I will tell you,due to tolerances,the cartridge head does not exactly rest on the breech face when stopped up against the headspace feature.We try to make the ammo just a bit short.It might be .002,or .004,or .006 for the M-1/M-14.
We want a little CLEARANCE between the CASE HEAD and the BREECH FACE so the gun can lock up securely without interference.
Guess what the CLEARANCE btween the HEAD and the BREECH is called?
HEAD CLEARANCE
You want SOME.but not TOO MUCH.
How do we do that? There are several methods to measure and control it.
Its easier to do than it is to explain because I'm not allowed to use the words "Cartridge Headspace" as opposed to "Chamber Headspace",the difference being "Head Clearance"
And it seems every time someone asks,we re-write the whole discussion.
I don't know why.
