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Old December 27, 2012, 07:57 PM   #1
Bart B.
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What Part of the Case Body Around its Pressure Ring Expands the Most When Fired?

I've seen in print and heard folks say the following; pick one as your choice:

1. The top of the case where there's lots of clearance as its bottom rests in the bottom of the chamber when its fired.

2. The thinnest part of the case as that expands the most from pressure.

3. The part of the case where the extractor presses it off center in the chamber.

4. The part of the case opposite where the extractor presses it off center in the chamber.

5. The part of the case that's at the widest point in the chamber; all chambers are not perfectly round.

6. Some other part; explain: _____________________________ .

Note 1; the pressure ring's typically referred to as that part about 1/16th inch or so in front of the extractor groove, belt or rim; depends on case type. It's usually more/higher/bigger at one point around it than the rest.

Note 2: consider those cases whose pressure ring bulge is the same height above the case body right in front of the extractor groove; even all the way around.
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Old December 27, 2012, 09:07 PM   #2
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Depends somewhat on the chamber size itself but of the list the one I'd pick would be #4, the part of the case opposite of where the extractor pushes it off center. Also this #4 is effected by number 5 and is more evident in a generously sized chamber. My experience is that the case expands most just in front or where the solid case head and thick tapered brass walls stop and the expansion is more on the opposite side of the case where the extractor pushes the case against a maximum sized chamber. Not so much in a tighter chamber. So I pick #4 which is effected by #5. Just my opinion and could very well be wrong?
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Old December 27, 2012, 09:43 PM   #3
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Just above the web, in my experience with rifles anyway when cases start to show a light ring just above the web area that's the end of that cases life, load and shoot it again after you notice the light ring and it will be cracked if not totally seperated, so far I have never had one totally seperate but close enough I have finished breaking them by hand, I've only went this far a couple times til I learned what to look for, and when or if I see that light ring I discard them.
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Old December 27, 2012, 10:18 PM   #4
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I think you may have asked the wrong question.

Various parts of the case expand, and some of them compress again once pressure decays. If that weren't the case (no pun intended), you wouldn't be able to extract the case.

If the issue, however, is incipient case failure in the case (there we go again) of bottleneck rifle cases, the issue is stretch, not expansion.

What happens is this:

You chamber the case in a chamber that is larger than the case (else the bolt wouldn't close).

When the firing pin strikes the primer, it pushes the case forward. The greater the headspace, the further forward it goes.

As the powder combusts, gas pressure causes the neck of the case to expand against the walls of the neck area of the chamber, causing a gas seal.

As the bullet now begins to move, pressure tends to push the head and web of the case aft, into the breech face. Since the neck is still fixed in the forward end of the chamber, this force tends to stretch the case in the area just above the web.

Repeated firing, sizing, and firing again causes repeated stretching, with each cycle thinning the walls of the case just above the web. If the process is unchecked, the case will fail at that point. (Before this happens, you should see a ring, brighter in color than the rest of the brass) just above the web.)
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Old December 28, 2012, 01:14 AM   #5
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In a 6.5x55 swede with a "generous" chamber I found Winchester brass expanded about halfway up the brass body.

Which convinced me to neck size 6.5x55 Swede in milsurp chambers. The brass is just fine as long as it can expand just a little bit and be supported by steel, but a full length resize gave me more than three thousandths difference, which is "excessive" reworking of the body of the cartridge in my opinion.

In rounds that have "cooked off" on the ground, the brass usually bursts at the shoulder. At least in 5.56 blank ammo with a crimp. Evidently the force of opening the crimp is greater than bursting through the shoulder like an overfull balloon.

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Old December 28, 2012, 10:43 AM   #6
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I'm referring only to the pressure ring; about 1/16th inch in front of the extractor groove at the case head. And only the place around it where the case pressure ring has expanded the most. No other part of the case was considered in my query. My original post has been so edited.
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Old December 28, 2012, 11:13 PM   #7
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I measure the extractor groove and spin the case around looking for run out.
Then I fire the case.
Then I measure the extractor groove again and spin it around looking for increase anywhere around the circle.

This is so much better than anything else for cartridges with loose primer pockets being the weak link in a strong gun.

1) In 6mmBR I work up until the primer pierces with CCI450 primers.
2) In 38 special I do not work up loads until the cases are sticky with fast powder, because the chamber may split first.

Those two don't count. But with large Boxer primer Mauser case heads [22-250 to 35W] and belted magnum heads [7mmRM], and .222 parent case heads [.223], this is what works best for me.

Waiting for primers to fall out or feeling primer insertion force or measuring pressure with a strain gauge is just not as good an indicator.

REAL engineers ask the question over and over, "What are you trying to accomplish" until they wear people down to the truth.
Attached Images
File Type: jpg Dial calipers on extractor groove 12-28-2012.jpg (76.4 KB, 13 views)
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Old December 28, 2012, 11:44 PM   #8
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A useful explanation of technique and results of measuring case head expansion can be found here.

When I see folks using calipers to measure something that requires 0.0001" accuracy, I have to wonder how meaningful their data is.
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Old December 29, 2012, 04:16 AM   #9
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Jeep2,
That link leads to Denton Bramwell type information.
He is a strain gauge / saami pressure for strong rifles person.
If that is what you are, stop reading what I am writing.
He and I are both electrical engineers.
He is a "do it per procedures" type and I am a "get 'r done" type.

In this pic I have uploaded, one can see I have measured the threshold of extractor groove expansion in .308 cases.

If you think I need more resolution than a dial caliper, stop reading what I am writing.

Per Vernon Speer in 1956, when one finds the threshold of brass change, one should back off 6% powder charge for the max load.

I have found that with Hodgdon extreme powders and my own rifle, I can back off 4% powder charge and I have safety margin for long brass life over a range of variables including; temp, brass volume, powder throw variation, primer variation, dirty chamber variation, bullet seating variation, ect.

CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
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Old December 29, 2012, 09:28 AM   #10
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This is from Varmint Al site

http://www.varmintal.com/a243z.htm
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Old December 29, 2012, 09:44 AM   #11
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Roper, good link to Al's site, but I don't find anything about where around the pressure ring is the expansion the most nor where it's at when fired relative to the chamber. Please tell me where it's at if it's there.
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Old December 29, 2012, 10:26 AM   #12
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I think when you look at his CHE toward the bottom of the article. this is his comment

To determine the amount of permanent case head expansion, node 21905 was selected and the increase in diameter at this location is plotted. The plots show the case head expansion during firing and finally, the permanent increase in diameter after firing. The location at node 20260 typically has about 0.0005" less expansion.
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Old December 29, 2012, 12:04 PM   #13
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Bart B.

Your restated original post has me curious to mark my brass for my next slow fire event.

If we think through the problem as you have restated it becomes an interesting experiment. If we assume that the pressure ring is unsupported by the chamber we can look at it this way:

The firing pin falls, the primer detonates, the powder burns. Now the gas is pushing the brass outward, the case is stretching backward, at peak pressure the "pressure ring" is as far back as it is going to get with only the brass to contain the pressure.

If the chamber is out of round, it will expand to fill that out of roundness, from the thinnest part of the brass towards thicker parts. This out of roundness can come from further forward in the chamber.

If the brass is out of round, or thinner/weaker in one spot, it will expand more to exploit that weakness, expanding until the brass is in contact with steel or until the pressure subsides as the bullet moves down the bore.

If the chamber is perfectly round, and the brass perfectly uniform, the extractor pressure will add stress to that one side, and it would expand more (although I doubt this is a measurable phenomena as the spring pressure on the extractor is nowhere as stable as chamber or bolt head steel).

If there chamber is perfect, and the brass perfect, and no extractor pressure, then gravity would be pulling the top and compressing the bottom, so we would expect more expansion on the top.

Like water flowing to the lowest point, pressure always tries to take the path of least resistance. However with the amount of variables involved, I doubt that any two rifles would produce the same brass flow.

If I had to rank factors in what I perceive to be importance it would be: Chamber uniformity, brass uniformity, extractor pressure, gravity, in that order.

Now I have to mark my loads to measure reference the top of the camber and bring along a micrometer to see if I can find a difference in the pressure ring roundness.

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Old December 29, 2012, 01:53 PM   #14
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old roper, those two nodes you referenced in Al's page towards the bottom are in the length wise axis of the case. I'm referring to the pressure ring height around its circumference on the case relative to the circumference at the front of the extractor groove; the pressure ring's highest point above the case body behind it. It's easy to see by watching reflected light from the case as you spin it; there's less bulge on one side of the case than opposite it.
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Old December 29, 2012, 04:53 PM   #15
Bart B.
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Jimro, your comment:
Quote:
If there chamber is perfect, and the brass perfect, and no extractor pressure, then gravity would be pulling the top and compressing the bottom, so we would expect more expansion on the top.
... won't happen. The front of a rimless bottleneck case has exactly the form and shape as the front of the chamber. Doesn't the front end of a headspace NO GO gauges coned shoulder center perfectly in the chamber's coned shoulder when the bolt tries to close on it?

The case shoulder will perfectly center in the chamber shoulder when the firing pin makes the round fire; 98% of the case Body doesn't touch the chamber wall at all. At the back end, it could be anywhere, but often is pressed against the chamber wall by the extractor if it is putting pressure against the case at its extractor groove. If there's no force applied at the extractor the back end of the case could center most any place, especially if it's twisted by contact with the bolt face when the bolt closes or gets moved off center by the firing pin striking the primmer off center on the primer's anvil driving it sideways a bit before firing.
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Old December 29, 2012, 06:07 PM   #16
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Quote:
Bar:
The front of a rimless bottleneck case has exactly the form and shape as the front of the chamber.
Like a tapered tool pulled into a tapered hole, it should center within ~.0001" at the taper and have excellent axial alignment away from the taper.
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Old December 29, 2012, 06:46 PM   #17
Jimro
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Bart B.

I agree that in a perfect chamber, with perfect brass, that the bulk of the cartridge will be an "unsupported tube."

What effect does gravity have on an unsupported tube?

Think putting the round in the chamber by hand, then using a bolt with no extractor or ejector on the bolt face, where is gravity creating stress in the brass?

There is no contact with the bolt face to add support to the case head, so my original summation of perfect brass, perfect chamber, no lateral pressure stands.

Sorry if I didn't make that clearer.

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Old December 29, 2012, 07:38 PM   #18
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After the pressure ring expands then case tapers back to the shoulder. If you mark the length from start of case body to pressure ring then mark that same length forward toward the shoulders that dia is just little larger on my cases.

Height of pressure ring varies since you get that from start of case body to rim which that thickness varies.

If some tell me that case taper more toward the start of the case body I don't have chambers like they do same with height of pressure rings.
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Old December 30, 2012, 06:25 AM   #19
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Jimro, in-line ejectors in the bolt push the loaded round forward until its shoulder stops centered in the chamber shoulder. If that don't work the smack of the firing pin does it. So the rimless bottleneck round's well centered up front where it counts when it's fired. Most extractors push the back of the case against the chamber wall so there's clearance opposite that point.

Take several primed empty cases, blacken their shoulders with a marker, then chamber them and pop their primers. Then look at the marks all the way around them from their impact as the center in the chamber shoulder. Use a case headspace guage to measure the cases both before and after firing. See how much the shoulder's set back from firing pin impact. That happens before the primer fires. How much depends on firing pin strength, pin protrusion from bolt face, primer cup hardness, case & chamber shoulder area and angle as well as the friction between the case brass and chamber steel.

Regarding a chambered round with no extractor or ejector putting pressure anywhere on the case, the case head could be anywhere when it fires. When the firing pin smacks the primer, it's possible that an off-center strike could push the case head sideways as the dimple was off center on the primer's anvil. And the way the case behaves as it's shoulder drives hard into the chamber shoulder may also move the back end of the case laterally. To say nothing of how the bolt head may center relative to the chamber when it's closed. The back end of a round so chambered is free to move with whatever other forces get applied to the case before the primer fires. I doubt all the rounds' back end is at the same place for all shots fired as long as the pressure ring's diameter is smaller than the chamber at that point when the round's chambered.
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Old December 31, 2012, 12:07 AM   #20
Jimro
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Bart B.

Everything you wrote is true. That is why we eliminated those variables from our thought experiment.

In real life it isn't hard to use electronic ignition and remove the extractor and ejector, which is what one would have to do in order to answer your question properly.

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Old December 31, 2012, 08:07 AM   #21
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Jimro, or measure case wall thickness and compare its thinnest part to where the case was oriented in the chamber when fired.

That can be done by using the primer's finger print from the firing pin tip as a reference. Slowly extracting a fired case from the chamber then marking it's top point with a marker will indicate where the top of the case was in the chamber when it fired.
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