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Headspace -vs- Case Stretch

 
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  #1  
Old 06-30-2012, 03:16 PM
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Headspace -vs- Case Stretch

Other than the intro section, this is my first post here. This may have been covered here, but I can't find it....and this may be better in the reloading section! And, if it has, a link or search word(s) would be appreciated.

QUESTION: In normal terms......would excessive headspace stretch the case wall nearer the case head or nearer the shoulder?

I'm one of those "need to know" kind of guys. Not a "know it all", but strive to know more and more about anything and everything (...well everything about shooting and ballistics, etc. for sure!). Unfortunately I don't have the means to discover the answer on my own.

It is my understanding, when pressure grows, the case is pressed up against the chamber walls, then to the shoulder, then to the case head and finally to the neck. If that were true, a chamber with a field grade headspace would stretch the case wall (closer to the case head), more than a chamber with a No-Go headspacing, considering both chambers were of equal size!

I understand this effect could be relative to the caliber, load, chamber, etc. My question is in reference in general terms, although I'm always open to being schooled on those determining factors which may or may not effect the results of any given rifle. And, regardless of my level of knowledge or ability, skills, I'm not offended if you want to get real basic with your contribution post.

Best Regards....Eagle Six
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Old 06-30-2012, 04:38 PM
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Re: Headspace -vs- Case Stretch

I'll be keen to hear the real answer, but I have a couple of thoughts.

First, thiner material tends to streatch more. That would be towards the shoulder/neck.

Second, unsupported material streatches more. If your lugs are hammered back and the excessive headspace is reflected in excessive space between the bolt nose and chamber, then the pressure ring ahead of the web will expand excesssively outwards.

So, I think it may depend a little on the individual chamber and brass.

...just a hunch, but not sure. Hopefully someone knowledgeable will comment.

-- richard
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Old 06-30-2012, 07:29 PM
KRP KRP is offline
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Re: Headspace -vs- Case Stretch

Excessive headspace or improper sizing of the brass(pushing the shoulders back too far) leads to case head seperation. The case is pushed forward in the chamber by the firing pin then swells and sticks to the chamber, the brass stretches just above the case head as the excess clearance is now between the bolt face and case head.
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Old 06-30-2012, 10:16 PM
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Re: Headspace -vs- Case Stretch

Quote:
Originally Posted by KRP View Post
Excessive headspace or improper sizing of the brass(pushing the shoulders back too far) leads to case head seperation. The case is pushed forward in the chamber by the firing pin then swells and sticks to the chamber, the brass stretches just above the case head as the excess clearance is now between the bolt face and case head.

+1

Perfect answer and a good description of what realy happens when excessive head space
is present for one reason or another.

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Old 06-30-2012, 10:53 PM
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Re: Headspace -vs- Case Stretch

Quote:
Originally Posted by KRP View Post
Excessive headspace or improper sizing of the brass(pushing the shoulders back too far) leads to case head seperation. The case is pushed forward in the chamber by the firing pin then swells and sticks to the chamber, the brass stretches just above the case head as the excess clearance is now between the bolt face and case head.

+2

The combination of the firing pin strike and the jetting action of the firing primer against the inside of the primer pocket will push the case as far forward in the chamber as possible and the case will stretch rearward to the bolt face. If you are shooting an unbelted case the case shoulder and chamber shoulder is the contact point and there is not much stretching since the gap between the case shoulder and chamber shoulder on new brass is in the range of .001" to .011" (largest I've measured). But on a belted case the belt will stop the case from moving forward in the chamber and that gap will be from .015" to .040"




with most being around .020"

Still belted cases will stretch at the pressure ring just above the solid case head. The excessive stretching on belted cases is why they are more prone to a case head separation. The initial stretch is much greater than on an unbelted case. Also why it is important to headspace a belted case on the shoulder as soon as possible.

IMO the neck swells first then the case body and then stretched back to the bolt face. The neck needs to seal the chamber otherwise there would be a lot of escaping gas around it that would discolor and cave in the thin neck brass.
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Old 07-01-2012, 01:02 PM
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Re: Headspace -vs- Case Stretch

Thanks folks for all the replies and valued information.



Quote:
Originally Posted by woods View Post
IMO the neck swells first then the case body and then stretched back to the bolt face. The neck needs to seal the chamber otherwise there would be a lot of escaping gas around it that would discolor and cave in the thin neck brass.
Considering a typical caliber such as a 308 or 06, please take this statement and change what you think is incorrect.....

"The sequence of case expansion would be the neck, shoulder, body, head/base."

Would it be accurate to say the edge/radius* between the neck and shoulder was the first part of the case to feel the chamber pressure, then out along the neck, as it also progresses out along the shoulder, then the edge/radius* between the shoulder and the body, down the body and to the head/base?

Or, would the very first part of the case to pressurize against the chamber start at the neck mouth and proceed back?

* I've searched and have failed to find the proper nomenclature, if anyone has a diagram or definition and proper reference name, I would appreciate it.

Best Regards.....Eagle Six
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Old 07-01-2012, 10:14 PM
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Re: Headspace -vs- Case Stretch

For a .308 Win. case 2.005" long and charge giving normal, safe maximum pressure with a brand new case headspace at about 1.628" fired in a SAAMI spec chamber with 1.631" chamber headspace, impact from the firing pin will force the case shoulder hard into the chamber shoulder setting the case shoulder back a thousandth or two as the primer fires and starts the powder burning. The case neck and shoulder will expand first as they're the thinnest, then the body starting right behind the shoulder and working back until the pressure ring in front of the extractory groove expands against the chamber. The primer backs out of the case a little bit when this happens. The case head also moves back as the back half of the body expands until it stops against the bolt face pushing the primer back into the case flush with the case head. This case expansion pulls brass out of the case neck back into the shoulder and the fired case is a few thousandths shorter that when it was new; in this example, maybe 2.003" long.

Full length sizing that fired case makes it grow in length; how much depends on how much its diameters get smaller an how much the case shoulder's set back. It usually ends up a thousandth or more longer than when new; in this example, maybe 2.006". Neck only sizing the fired case lengthens the neck only a few ten-thousandths of an inch.
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