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Old 05-22-2010, 01:17 AM   #1
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Default So- just what the heck IS headspace?

We have all heard the term- and probably read dire warning about swapping bolts or barrels- and being sure that a smith has checked headspace on a used gun- but what IS this thing? And why should I be concerned about it?

As an oversimplification of the term- it is how much free space exists BEHIND a chambered cartridge. Too little, and the bolt or slide will not close, or a cylinder turn. But when headspace is excessive, very bad things may be about to happen.

When a cartridge is fired, the firing pin pushes forward on the cartridge. The rapid expansion of gasses from the burning powder causes the cartridge case to expand- gripping the walls of the chamber, and forming a gas tight seal. As pressure drops, the case "rebounds" and shrinks, allowing it to be extracted. However, between those two moments, the bolt, slide, or face of the recoil shield is supporting the head of the cartridge case, and keeping it from stretching away from the forward part of the case that is locked to the chamber walls.

Some firearms can have headpace changed by repair or replacement of key parts, but some are well known for developing headspace problems due to wear and use. Perhaps best known is the SMLE- the Short Magazine Lee Enfield, and it's sucessors, the No.4 Mk1, and the No. 5 carbine.

The design of the bolt and reciever, with locking lugs at the REAR of the bolt, creates a much longer section that holds headspace to designed tolerances. However, the rifle was designed so that as it wore, bolt heads could be replaced with sucessively longer heads, countering the stretching effect- IF the rifle is properly maintained.

The photo below shows 2 fired .303 Enfield cartridges- a normal one, and one from a rifle whose owner was invited to clear the rifle, and remove it from the range. This was new, commercial .303 ammo. In addition to the split that goes more than halfway around, the primers were falling out of their pockets. The owner had merrily fired 10 rounds before he was stopped. In the photo you can see where the casing has been reduced in size between the rim and the crack, due to stretching the brass. This rifle was venting hot, high pressure gasses to the rear, and the owner was not injured due to the design to route gasses away from the shooter. In some cases, the violent SLAM of the case head against the bolt may shear components, blowing the bolt back at the shooter.

It is a wise move to have firearms checked that are older, or had repairs to barrels, bolts, or slides- and to take a look at your fired cases for signs of problems- stretching, cracks, or primers that just don't look right.
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Old 05-22-2010, 06:10 AM   #2
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An important bit of information to be sure but it might be worthwhile to go ahead and make sure the information is correct. You are correct in stating that your explanation of the term is an oversimplification. The distance between the breech face and a chambered cartridge is actually head clearance, I know that many refer to it as headspace and I have done so myself. What's more, I will likely do so from time to time in the future but I'll do so with the knowledge that the definition police will be all over me!
Headspace, insofar as the firearms trade is concerned, is the dimension from the breech face to a designated point in the chamber. In a chamber for a rimmed cartridge (like a 303, 30/30, or a 30/40 Krag) the headspace measurement is the distance from the breechfaceto the front of the rim recess. For the cartridges mentioned the minimum chamber measurement is typically .063".
If the chamber is for a rimless, bottlenecked cartridge (30/06, 308, 270, etc.), the headspace dimension is the distance from the breech face to a line on the shoulder which measures a given diameter. In some cases, this line will be where the shoulder is 0.375" in diameter. This line is known as the "datum line" and is the reference point for all measurments on that particular cartridge. The headspace figure for the 30/06, for instance, is 2.0487" and the datum line is at the .375" diameter point on the shoulder.
If the chamber is for a belted magnum cartridge, the headspace dimension is the distance from the breechface to the front of the belt recess. The minimum measurement is typically .220".
When we are speaking of the cartridge itself, the measurment from the base or head, to the established point is also known as the hedaspace measurement. Generally speaking, the maximum cartridge will be equal to the minimum chamber.
So that is the definition of headspace as a noun but we often hear the term used as a verb. "The 30/06 headspaces on the shoulder", or, " the Ackley Improved chamber is designed so the factory cartridge headspaces on the neck/shoulder juncture". Headspace , as a verb, is already a misuse of the term. If we understand the meaning though, I think it's alright.
The difference between the measurement in the chamber or the rifle and the cartridge, is head clearance but is what many have come to describe as headspace. Since this is understood, it may be alright as well.
There is always a certain amount of head clearance built into the rifle/cartridge combination and because brass is able to tolerate some deformation, this is not a problem. If the clearance is excessive however, the brass will stretch past it's elastic limit (the limit at which it can return to it's previous shape and size) and it will stretch permanently. In a worst case scenario, it will separate.

In the post above, the Lee Enfield rifle was singled out as one which will develope headspace due to wear and use. While rifles may develope excessive headspace due to wear and use, it is not a common occurence and this includes the Lee Enfields. The truth is, many Lee Enfields have excessive headspace because it was built into them. That the bolt deflects and the receiver stretches under the stress of firing makes matters worse and head separations are not uncommon. The maximum headspace measurement for the rifles was generous to assure function under battlefield conditions and it worked. When a Lee Enfield rifle has excessive headspace, it is not a reflection of any design shortcoming but is a reflection of the British concept of what was important at the time.
I mentioned the Ackley Improved cartridges. Headspace on the Ackley cartridges is, as stated, based on the distance to the neck/shoulder juncture. One reamer maker is attempting to standardize headspace dimensions on these cartridges based on a datum line as in other rimless cartridges. This opens a real can of worms and it is questionable whether or not it should even be done. I may get into this further at some later date but for now, I think I have said enough! GD

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Old 05-22-2010, 10:34 AM   #3
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Greydog- very good post. As I said- an oversimplified definition.

The SMLE that was mangling that brass scared the poop out of us- it was not just "on the edge", but well beyond the edge, and picking up speed quickly.

The thing that I suppose has surprised me several times is a gun that does far more than it was designed to do. Have seen folks do dumb things- wrong ammo, BAD hand-loads, shooting a gun without parts that it REALLY should have had in place, even obstructed bores- and the gun did not kill the shooter. Have seen a S&W Model 10, 5 inch pencil barrel- with 6 bullets wedged in the barrel- first shot was a squib load.

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Old 05-22-2010, 01:16 PM   #4
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Yes, I have seen the same thing many times. In some cases the shooter quit only because the cylinder would no longer turn. I guess he figured he was just missing the target.
The Lee Enfield is not particularily strong nor is it all that attractive; what it is, is dependable. In the case of a severe overload, the action would fail but would do so in a way which was quite unspectacular. The bolt would deflect upward, the receiver would flex downward, the case would fail and the gas was vented. The rifle would subsequently be unservicable but the shooter would likely have suffered no injury. UNder the same circumstances, a Springfield would grenade.
Looking at the pictures of the brass and judging by the corrosion on the separated case, I would guess that was a fairly old cartridge. It is possible that the brass had hardened with age and had become brittle; making it unable to stretch without fracturing. The other case, though it may show signs of incipient separation, was able to stretch without cracking.
When a case does separate, there is some advantage to a rimmed case. The rim acts as a baffle and is able to contain or deflect the escaping gas. To a somewhat lesser extent, a belted case does this also; just not as well. With a rimless cartridge, the rifle design has to get by with no help from the case. GD

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Old 05-22-2010, 01:51 PM   #5
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Old 05-23-2010, 05:40 AM   #6
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If you need pictures, it's a good thing you were able to find some! Those pictures are indeed illustrative of headspace if the term is used correctly. Of course, the reality is that each time a discussion of headspace gets de-railed by what is, in essence, quibbling over semantics, the value of the discussion is lost. C3shooter brought up a subject which is of some concern to all shooters and I sort of derailed it.
Call it headspace or call it head clearance, if the chamber is too long in it's headspace measurement or the cartridge is too short, a condition exists which can be dangerous and is certainly to be corrected.
Handloaders frequently introduce excessive headclearance into a rifle/cartridge mix during the sizing operation. That many gunsmiths and others call this headspace when the term is technically incorrect doesn't really matter. If the term is understood in that context, it will work out. When my Grandfather blew the extractor out of his 270 Gibbs by firing 270 Winchester factory ammo in it, Rocky Gibbs told him he was firing a rifle with grossly excessive headspace by using factory ammo in it. Whether or not the term was correctly used, Grandpa understood the lesson; so, Rocky's improper use of the term "headspace" was of no consequence, he got the message across. I suppose, if a term is used in a different way often enough, it's use in that way becomes legitimate. I'm a gunsmith though, not a linguist, so I can't really say. GD

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