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Old August 26, 2011, 03:40 PM   #1
GregInAtl
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Headspace

Can someone explain to me the concept of headspace and how to determine it and what to do if it's not correct.
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Old August 26, 2011, 04:05 PM   #2
mehavey
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Loosely-defined, headspace is the "rattle-room" that a cartridge has to move within the chamber once the bolt is closed.

For bottle-neck cartridges: Chambers are factory cut to a specified dimension from bolt face to mid-shoulder (± a few thousandths) depending on the cartridge. If the cartridge case is sized to those exact dimensions, the assembly has "zero" headspace. If the case is sized shorter than the chamber by (say) 0.005", then the excess headspace is that 5 thousandths of an inch.

Rifles are designed to work best with a few thousandths clearance of headspace. But many thousandths causes the cases to stretch excessively upon firing, which after repeated reloading may cause stretching beyond the ability of the brass to handle it, and the case separates near the head. -- bad juju.

Headspace is best determined by a gunsmith's use of (3) solid metal headspace gauges machined to exact dimensions.

The "GO" gauge is the shortest and MUST allow the bolt to close. The NO-GO gauge is 6-8 thousandths longer and the bolt is NOT supposed to close on that one. The chamber is therefore between the two dimensions and properly cut to spec.

If the bolt does close on the NO-GO gauge, then the "FIELD" gauge (several thousanths longer still) is used. If the bolt won't close on that gauge, then the rifle is considered safe to shoot w/ factory ammunition, even though not technically in spec.

If truly excess headspace is discovered, the usual solution is to remove the barrel and machine it to where it will set/screw back into the receiver an additional amount. (Others may know better whether a full turn is req'd for barrels not having a front sight), but usually the gunsmith will then re-cut the (now shorter) chamber to correct headspace specs.

Last edited by mehavey; August 26, 2011 at 05:20 PM.
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Old August 26, 2011, 06:32 PM   #3
GregInAtl
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Doesn't sound like anything to be concerned about when reloading pistol ammo. I saw the term used in the Lymans 49th but a lot of times they don't differentiate between what applies to rifles and what pertains only to pistols.

Thanks for the info
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Old August 26, 2011, 06:43 PM   #4
mehavey
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In "pistols" (autoloaders), the case 'headspaces' on the case mouth. You are correct that it rarely becomes an issue (unless you overcrimp the mouth and it thereby misses the seating against the stop 'ledge' in the chamber.)

Revolvers headspace on the rim and are even less of a concern.
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Old August 26, 2011, 08:28 PM   #5
wncchester
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"Loosely-defined, headspace is the "rattle-room" that a cartridge has to move within the chamber once the bolt is closed."

Excellant word picture. People too often get into how it's measured without giving a clue what the heck it really is; no wonder so many folk get confused about it!
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Old August 27, 2011, 09:14 AM   #6
Hammerhead
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Sometimes in auto pistols if the chamber is a little long and the brass is on the short side, the case will actually headspace on the extractor. It happens more often than people realize.
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Old August 27, 2011, 09:30 AM   #7
Unclenick
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I'll just add in that historically a lot of early cartridge designs were rimfire, so the rim was the head. Thus, the total space made available to accommodate the rim was literally the headspace. I once referred to it as the rattle room, too, but another member pointed out that, once the cartridge is in place, the rattle room would be only the excess headspace.

Today, as mentioned earlier, we refer to headspace as the measured number from the breech to whatever it is that ultimately stops the cartridge moving further forward in the chamber. Rim, belt, shoulder, case mouth, or extractor are most commonly the stop.

To throw one more in, for lead bullets in self-loaders, I often seat the bullet out far enough to stop against the throat of the bore before either the case mouth finds the end of the chamber or the rim finds the extractor hook. This is headspacing on the throat, though it is more commonly called headspacing on the bullet (don't know why). It reduces leading and improves accuracy with soft bullets. I don't find it makes any difference to jacketed bullets.
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Last edited by Unclenick; August 27, 2011 at 09:35 AM.
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Old August 29, 2011, 06:06 AM   #8
F. Guffey
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"Excellant word picture. People too often get into how it's measured without giving a clue what the heck it really is; no wonder so many folk get confused about it!"

Uncle Nick, you can remember when every answer pertaining to head space always ended with the loudest member shouting "Well, you see, there is this here datum line up there and that is how they do it" They did not know what it was, they did not know where it was, they did not know how to find it and they could not use the information provided with chamber drawings to make a tool to verify chamber dimensions, and for the most part they are still pushing tools made by those that understand the datum is not a line, and, 'WOOO' be onto anyone that does.

(Again) ,A collector, resource, builder of 03s, 1911s and M1 Garands was needing help with head space on a Rock Island period correct 1911, he did not have a head space gage, he had 30+, anyhow, I purchased a mill from him and was loading it when the conversation got around to dysfunctional forums, so I ask him to explain, he did, then I informed him "This is your lucky day" I check head space on any chamber 3 different ways without a head a space gage, before I could get started he opened a drawer full of head space gages, I reminded him I do not use head space gages but with his permission I would modify one of his go-gages to a go-to infinity gage meaning he could use it to check head space on a 30/06 chamber from .005 over a minimum length/full length sized case to infinity in thousands. Even though the go-gage is useless he chose to leave the gage a 'go-gage'., making and modifying tools is not for everyone because when the bolt closes the chamber gets dark for most.

Anyhow, he had an average head space of .0075 when checked with a new box (20 rounds) of Remington 30/06 ammo, along with determining the length of the chamber it is possible to determine the length of the case from the head of the case to it's shoulder, the difference between the 20 cases in length was .00075, and I thought that was outstanding, he was stuck with reducing the length of the chamber by setting the barrel back or changing barrels or changing bolts. He had no less than 80 Springfield bolts, but, only one was straight handle, I have no less than 30 with one straight bolt, I offered to check the effect each bolt had on off setting head space and I offered to check his spare barrel, back to the beginning 'period correct' he only had one bolt and one barrel, setting the barrel back with this crowd would not have been a problem, but for an improvement of .0025 head space?

I explained to him the dysfunctional forum that always turned ugly when the head space question was asked by reminding him of an old Hindu parable.

http://joyfulministry.org/humorg.pdf

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