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Old December 26, 2013, 12:11 AM   #1
std7mag
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Headspacing help...

While looking at some SAAMI drawings I noticed that my preconceived notion that the "critical headspace" measurement was taken to the end of the neck where the angle for the "bottle necking" started.
Looking at the 30-06 as an example it shows that measurement at 2.1086" from the base and is .3397" in diameter.

Closer to the base at 2.0526" it lists the "head space measurement" , which is actually on the face of the angle(bottle) where the diameter is .375" for both the cartridge and the reamer.

Is there a set correlation between the length of the angle(bottle) or is the measurement at the whim of the cartridge designer?

I guess the question is where does the designer designate the head space measurement?

Thanks!!
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Old December 26, 2013, 12:30 PM   #2
mehavey
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[30-06 Spr] headspace is measured where the angled shoulder intersects w/ a 0.375" diameter circle.

Since that shoulder starts at 1.940" from the base, at a point where the case diameter is specified
to be 0.4410", and then angles toward the neck at 17.5°, simple geometry kinda nails down
where that headspace "circle" meets things

Last edited by mehavey; December 26, 2013 at 01:46 PM.
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Old December 26, 2013, 05:19 PM   #3
std7mag
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Mehavey,

While this is all well and good, it doesn't exactly answer my question.

What/who designates that this is where the headspace measurement will be?

Someone at SAAMI? The designer of the cartridge? Or Bob down the street??? lol

Also why would someone designate it at that measurement? aka the reason why it is, what it is???

Egads I sound like a 2 yr old...
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Old December 26, 2013, 05:51 PM   #4
Jim Watson
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As I understand it, the designer gets his specifications listed by SAAMI.
Of course if it is a wildcat or proprietary cartridge, the headspace and other dimensions are entirely the maker's.

The .30-06 is a special case. The original mil spec, back in 1903 when rimless bottleneck cartridges were new, called out headspace to the intersection of the shoulder cone with the body. Since that is not really a sharp angle in chamber or cartridge, it was kind of an arbitrary number. Gauges were ground to give that length as though they DID have a sharp corner there.
You will see old publications listing .30-06 chamber minimum headspace as 1.940" measured that way.

Commercial bottlenecks have headspace figured to a "datum circle" about the middle of the shoulder. Since that is a physical location, it is easier to set up to specify and measure. The SAAMI minimum spec for .30-06 measured to a .375" diameter datum circle on the shoulder of the chamber is 2.0487".
Two different numbers measured two different ways give you the same chamber.

Different cartridges use different diameter datum circles, in proportion to the size to give a location near the middle of the shoulder. .375" is common, but a smaller round like .223 Rem datum is .330" diameter, the less tapered .308 Win is .400".

As to why, like Mr Mason said to Mr Dixon, we have to draw the line SOMEWHERE.
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Old December 26, 2013, 06:03 PM   #5
mehavey
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Quote:
As I understand it, the designer gets his specifications [registered] by SAAMI.
Of course if it is a wildcat or proprietary cartridge, the headspace and other dimensions are entirely the maker's.
Latest example is Bill Alexander's 6.5 Grendel.
At the end of 2010, Bill gave them the specs as he designed it.
SAAMI then listed/registered those specs as "standard" on 01-17-2011, and in their drawing made about a month later
http://www.saami.org/PubResources/CC...%20Grendel.pdf
(Note the use of the 0.350" diameter point on the shoulder as the reference/datum point.)



BTW: Note the list of SAAMI spec here:
http://www.saami.org/PubResources/CC...0-%20Rifle.pdf

Last edited by mehavey; December 26, 2013 at 06:12 PM.
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Old December 27, 2013, 05:28 PM   #6
std7mag
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Thank you,both!!!

Thank you both for your input!!

More math skills on my end needed!!! lol
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