Case Head Expansion

[wildcatter]

Hoot, I love this picture of your's. One reason is that it amply shows the measuring area, for case head expansion, which for the uninitiated is the shiny area about .200" north of the rim, about .100" north of the top of the extractor groove, at least that's where I measure mine.

Question: I read that primer study from James Calhoon you site and it has some really good stuff about primers. Did I read correctly, he seemed to be touting, the case head measurement area, to be exactly north of the extractor groove, the area that actually touches that groove, but is on the case body???

Taco, which area were you measuring the .003"-.004" expansion, using 1680 and those 350gr Interlocks? I assume you did a before and after comparison, correct? As I do my measurements, 3-4 thou is allot. Can you describe your methodology?

Lemme just say that measuring case head expansion is poor at best, for reading pressure signs. If for no other reason, the lot to lot changes of brass composition, throws the reading out of the window. Having said that, I keep track of the numbers, all the time, but I do so, by lot numbers. Poor as it is, case head measurements, I still find it useful..

..t

[Hoot]

Folks, WC is referring to the thread I put in my Information Depository which addresses Judging Pressure when Reloading. In it, I cite an article by a fellow who promotes measuring case head expansion at the solid part of the case head as opposed to the wall just above the web.

Tim, I actually was hoping you'd wade in on a this post I made in another thread WRT the method for measuring this phenomenon. It is reassuring to know I'm not going crazy because I felt I understood what I've been measuring the past two years and along comes the guy's article, which makes sense, but seems to fly in the face of what I thought we were supposed to measure. Measuring where he does is chasing a small change, using what for the average Joe would be tedious precision. As Tim said, the degree of change will rely heavily upon the other metals that are blended in the cartridge brass being the same from batch to batch. It's also going to vary depending upon the number of times the case has been fired and reloaded from work hardening. That does not even take into consideration the fact that with our mass produced chambers, fired brass can take on asymmetrical shape and just rotating a spent case will reveal diameter variations way outside the degree of precision that is employed in Jim's article.

Neither method is equivalent to Reading Tea Leaves, but the readings have a degree of subjectivity nonetheless. All that having been said. If WC says to measure it above the web (our historical approach) I accept that as a better method for predicting pressure conditions as best we can, over the method prescribed in Jim Calhoun's article.

Some other members, give that article a read and wade in. If you concur, I'll pull that Judging Pressure thread or rename it to reflect the other subject in the article regarding Primers.

Hoot

[wildcatter]

Ok then, I guess I read it right.

To measure the case head, immediately at the north end, of the extractor groove, is well, problematic for me. If one considers, that, that area is solid brass. In my mind, if you get any expansion, in that area, you are way-way-way (I may not have said WAY enough) over loaded.. No, as for me I want to read the signs, subjectively, as you say, in the area where the Die does not size the case and that area is typically .200" inch above the rim and is normally the area where the Web lets off for the sides of the cartridge.

Taco, I need you to weight in here, are you measuring the area, Hoot and I are describing? If you are and are really getting 3-4 thou expansion, then you'd better back off that load. I will except 1.5-2 thou, but then again I trust my mikes, doing it under controlled conditions and otherwise have a lab to control all of the conditions. I'd never take to the field with anything that expanded over .0015" (1.5 thou) and I normally see, "Less" than .001" expansion in factory fodder.

So Taco, what I'm looking for, is something amiss in your methodology. I don't believe under normal circumstances, you should be getting that much expansion with those 350gr loads, you've listed, in fact I think them somewhat underloaded, BUUTTT, we'd better bumps heads some here, before we make that Prognosis. What makes me think that you may be underloaded, is the lack of other signs. You might, assuming your methodology is up to snuff, be suffering from pre-detonation (as in a pressure wave event). The Gunny suffered from this using light loaded ww-296, in the 450LM. His cure was to indeed increase the powder amounts. But there are other ways to do this, like using Hoots Toppings (Hoot I can't remember, was it Barbeque Sauce or what was that medium. I'm thinking it was BullsEye Brand BBQ sauce..wink), for the powder, to otherwise, fill the case. Meaning you can, even though 1680 isn't normally associated with the phenomenon, under-load the case and get a big surprise, by detonating the powder and destroying your weapon or worst. Using a medium, like Cream of Wheat and there are better ones, to take up the excess case volume, that the powder isn't, etc. But this is just talk, and we need to talk about your procedures and thereby edify our masses..

..t

[Hoot]

Though it's not my style to proctor, a small caveat is in order. More reloaders own calipers than micrometers. Calipers, even high buck units, are not the best device for precisely measuring case head expansion. You can get very accurate micrometers for not more than a modest caliper. Look at the specs that came with your caliper, assuming they are honest and its not out of the ordinary for them to be spec'd +/- .001. Most micrometers easily hold .0001 and many can be interpolated to less. Certainly enough to go .00005, especially the ones with carbide tips. There are excellent values to be had on Ebay for used but certified micrometers, some for the cost of a box of FTX bullets. They may not be as convenient to operate as a caliper, but each device has it strengths and weaknesses. Well cared for micrometers last a lifetime as their mechanics are much simpler than a caliper.

Hoot

[TacoTaco]

I'll post up a reply and some data tonight after work.

[pitted bore]

The technique of estimating pressure was publicized mostly by Handloader magazine in the 1970s and 1980s, I think. The two authors I recall most heavily promoting the technique were Ken Waters (of "Pet Loads" fame) and Bob Hagel. Ken Howell may have done his share also.

Checking some back issues of Handloader, I found an article by Hagel that seems to show most clearly the
technique.

For rimless and rebated rim cases like the 450B, the technique requires a good micrometer, as Hoot notes.

Below are a couple of images with captions, scanned from "How to Detect and Control Pressure", an article by Bob Hagel in Handloader Number 91, May-June 1981, starting on page 18. The images are on page 21.
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From Handloader #91, 1981

press1.jpg (46.5 KiB) Viewed 18043 times

One image shows the measurement of the "pressure ring", which is the bulge that forms on most cases near the front end of the case "web". As the caption notes, this technique is not particularly reliable.
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From Handloader #91, 1981

press2.jpg (51.17 KiB) Viewed 18043 times

The other image shows a measurement over the solid part of the web, which is more reliable.

A couple of notes on the technique as outlined by Hagel and Waters: Before firing, one must mark the points at which the neasurements are to be made. More than one diameter should be marked, because cases do not expand uniformly around their circumference. At least three measurements should be made at each point, and these averaged, and then the two or three point averages should be averaged. (With the 450B's rebated rim, there is no problem with the rim interfering with the measurements as noted in the caption.)

The technique is not especially reliable; Tim is right on with his comments. Waters was very careful to limit its use to comparisons involving expansion of a factory load as a presumed maximum. He made the comparison with his selected load using a case from the same box as the factory load with the primer, powder, and bullet removed, and with his load's primer, powder, and bullet substituted.

Whether the technique is useful or useless is a topic for another thread.

--Bob

[wildcatter]

Way-To-Go, Bad Bob..

..t

[TacoTaco]

I remeasured the cases using the method described in this thread. Measuring .1" up from the top of the extractor notch, My pre fire data is .498-.500 (Based upon virgin brass from the same lot that I loaded). Post fired measurements are .500-.502. It looks like the .500 cases expanded to .501-502, with the .498-499 cases expanding to .500. However, since I used different methodology in my previous measurements, I cannot say with 100% certainty that all of the cases only expanded by .002 or less as I don't have a measurement on the virgin brass using the same technique.

Now that I have a standardized method of taking the measurements, I'll load up another lot of the same loads and measure pre/post fire accordingly.


TT

[Hoot]

While this table was part of the one for my annealing / range report, I thought it would provide some food for thought WRT measuring the pressure ring on the actual unsupported wall. I get greater readings than I think is normal, even with normal loads. As the article will show, these four runs were using a tame charge of 38gr Lil Gun, yet almost every one is incurring growth consistent with too much pressure. They were not stiff to shoot, that's for sure. The 200 FTX had the recoil of my 5.56 in conjunction with the recoil reduction techniques I employ. I know for a fact they were not over-pressured. It has been that way since I got the gun and that's why I suggest chamber dimensions as in loose vs tight might mitigate the method.



Interestingly, I took along a 1-off cartridge for another experiment that did indeed kick a 200 FTX up in the hot load range and it had stiffer recoil than the 38gr LG ones, but its pressure ring measured .5024. Smack dab in the range of the lesser loads. It did shrink length more at .008. That only adds to my consternation with this method. BTW, despite having lower velocity on average than the 250 FTX, the 250 Thompson Center Shockwave I tested had greater average growth for the same charge, COL and degree of crimp. Go figure...

Hoot

Edit: Edited to add case shrinkage to spreadsheet

[pitted bore]

... Measuring .1" up from the top of the extractor notch, ...

TT-
At 0.100" from the top of the extractor groove, you are taking measurements too far away from the head of the case. (By "top". I'm assuming you mean the mouth edge of the groove, as opposed to the edge closest to the head of the case.)

Take at look at the second photo by Hagel that I posted above. He's measuring about .020" to 0.030" from the top edge of the extractor groove. The measurement has to be made in the solid section of the web.

From the caption: "Jaws [of the micrometer] should not be placed very far forward, just far enough to measure full diameter ahead of the extractor groove, but not far enough to get a false reading from the taper of the extractor rim."
--Bob