Why is Head Space Important to Know


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HammeringHank
August 5, 2011, 04:42 PM
Both my Gerand and M1A have cards giving the headspace info. 1.***.
Please don't jump all over me. I'm a little thick headed, "DUMB"
I just don't understand.:banghead: Thanks, Hank

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ny32182
August 5, 2011, 04:47 PM
Basically, headspace will determine how much extra "space" there is when a round is chambered and the bolt is closed. Too little, and the fit will be too tight, too much, and it will be too loose and lead to stretched brass or casehead separation.

243winxb
August 5, 2011, 05:49 PM
http://www.brownells.com/.aspx/lid=12897/GunTechdetail/Gauging_Success___Minimum_Headspace_and_Maximum_COL A cartridge that is to long for your headspace will let the rifle fire out of battery. Meaning the action is not locked. If the cartridge shoulder has been pushed back to far on Full length resizing by more then .008" the brass may stretch enough to cause case seperation near the head or in the body. http://www.exteriorballistics.com/reloadbasics/gasgunreload.cfm

303tom
August 5, 2011, 06:07 PM
ny32182 is correct , basically so you don`t have casehead separation.

tarosean
August 5, 2011, 07:27 PM
There is a slight difference between the 7.62x51 NATO vs 308 win. Use the correct ammo

WNTFW
August 5, 2011, 07:39 PM
The chamber/bolt (Steel) is what contains the pressure. The brass cannot contain the pressure. The brass is the seal and the chamber the support for the seal. If the seal is too big it won't fit within the chamber. If the seal is too small it won't be supported and fail or weaken. It may fail after a few cycles in the case of reloads.

SlamFire1
August 5, 2011, 08:14 PM
Headspace is critical for safe functioning of a high power rifle.

The first consideration is the rifle case. The cartridge can only stretch so much than it will rupture. I believe I read it in Col Chin’s book that .006” is the standard maximum safe stretch. At least for a 20mm cannon round. After that the cartridge is highly likely to separate. The Brit’s called it case breakage if I remember right.

These pictures are examples of improper headspace. It is obvious that the shoulder was set too far back.

http://img.photobucket.com/albums/v479/SlamFire/Reloading/300WSMCaseHeadSeparation4.jpg


http://img.photobucket.com/albums/v479/SlamFire/Reloading/300WSMCaseHeadSeparation2.jpg


In terms of case breakage and strength there is some confounding data out there, from P.O Ackley. He was able to fire 30-30 Ackley Improved rounds in a M1894 without locking lugs and the case stayed put. I believe the chamber was very clean. Modern analysis of case stretching with a 308 shows how he got away with this.
http://www.thewellguidedbullet.com/m...al_studies.htm "Yielding of brass case walls in Chamber" by James A. Boatright


Mr Boatright shows that around 25 K psia a 308 case head will stick to the chamber wall and hold the pressure. However above that pressure the side walls will stretch and if the case head is not supported, the case will rupture.

One gentleman duplicated P.O Ackley tests with the 30-30 AI round. The AI round stayed put without a locking mechanism. However when he shot a regular 30-30 round, the case came out of the chamber at 1900 fps. If P.O. Ackley had used a 30-06 without locking lugs his handbook article would have had a different conclusion. But then, P.O was selling his AI rounds and trying to prove to the world that the reduced taper of the cases reduced the load on the bolt, therefore hot loading was Ok. I don't agree with the hot loading part.

Another issue with excessive headspace is peening. Too much headspace and the case head will slam into the bolt face. This is a shock loading and can peen revolver shields, bolt lugs, receiver seats. It will be aggravated by low friction cartridges. I lubricate the cases used in my M1a. Eliminates case head separations which occur from case stretching on extraction. However I only set my shoulders back .003”. I suspect if I were to have more than .006” headspace difference between case and chamber I might have experienced peening. As it is, I am on my third barrel and everything is good.

Everyone shooting a gas gun should use cartridge headspace gages. The Wilson type are easy to use:

http://img.photobucket.com/albums/v479/SlamFire/Reloading/ReducedWilsongagemeasuringnew308bra.jpg

This is the difference between go and no go. You will be very lucky indeed if your brass is sized correctly by the old “touch the shell holder and go a quarter turn”.

http://img.photobucket.com/albums/v479/SlamFire/Reloading/Wilsongagebetweengoandnogage308brass.jpg

There are issues with cases which are so long that they are a crush fit to the chamber. This is undesirable as it will cause an interference fit. The first problem is getting the bolt closed. You will read threads on this all the time, the guy does not use cartridge headspace gages to set up his dies and now he can't get the bolt to close. This will absolutely gum up a semi automatic mechanism. The second problem is getting the fat round out of the chamber. The interference fit will be worse after firing.

Long cases are invitations to out of battery slamfires in Garands/M1a’s. If the bolt has to stop and crunch fit the case to the chamber, you run the very real risk of an out of battery slamfire. These mechanisms have free floating firing pins which are tapping the primer all the way down. You want no delay to bolt closure in a Garand.

Headspace is something that must be controlled between the ammunition and chamber for safe reliable functioning of a firearm.

Kliegl
August 5, 2011, 08:45 PM
I see that Slamfire knows about his namesake.

For what it is worth, according to Springfield, the M1A is built to handle .308 Win chamber
pressure (which is the higher of the two) and is chambered to be able to accept both .308 Win and 7.62x51 NATO with some certain chamber size. Called them myself. I run .308 Win out of my HK91, M1A, and FNAR, and I run 5.56x45 (some .223 if I get a good deal) out of my AR and my HK93.

http://www.thehighroad.org/archive/index.php/t-343947.html

is informative.

Shadow 7D
August 5, 2011, 09:59 PM
Ok, too tight and the bolt won't be able to close on the round

too loose and the the case will burst and vent gas at 50,000 PSI in your general direction - not healthy for you

So, being inside tolerances is pretty important. Wiki actually had a pretty good explanation.

mshootnit
August 5, 2011, 10:16 PM
you need to know the headspace well in advance of purchasing the vehicle, that way when you adjust the seat you won't be bonking your noggin or spilling your drink.

Oh wait thats headroom!:D JK

Wasn't there a dude named Max Headroom in the 80's? Am I dating myself?:uhoh:

....and they put the cards in with the specific rifle to let you know the chamber dimension. This is to let you know that it was checked and is safe.;);)

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