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I hauled out the fired BMG brass to do a new cycle of case prep, and I noticed two partial case head separations.
I then checked each one internally, and found one more that was about to smile at me.
Till then, I'd had only one partial separation. This is now a total of 3 partials in about 100 rounds.
In the past when I checked sized brass headspace in this rifle, I did it with the firing pin and extractor in the bolt, because I have no idea how to take it out. Long story short, I ended up setting the shoulder too far back, and all my reloads since then were wonky with the end result being the case head separation.
A couple weeks ago I got my shoulder comparator, so I put it to use against the fired brass, including the ones that started smiling.
Then I needed an oversize BMG case to incrementally size till it fit the chamber. Years ago a friend gave me two fired SADF berdan cases, so I took those, cleaned them up and measured.
On my comparator and inches vernier, the trailing vernier numbers of the SADF brass was .868 at the shoulder. My fired brass and the smilers were .844.
I sized the berdan brass down incrementally till the bolt was just short of closing, then two thousands more to close and that was at .858.
In the previous setup I did, I was going by the feel on the bolt close, and I see now that unless the rim of the case is under the extractor when the bolt engages, there is a fair bit of resistance just to get the case head past the extractor to sit against the bolt face. This gave me a false "feel" to the closing resistance.
By working purely with closed / not closed, there's no feel. Common sense in hindsight.
Now things get interesting.
I was sitting with fired brass that I thought had a shoulder a bit too far back. I'd just neck size, no need to full length size.
As a test, I picked a fired case, measured .844, ran it through the full length sizer and it came out .858 . huh ?
I resized and check another 6 like that, various short shoulders came forward and all came out on .858 .
The last case measured .861, I sized it and it came out .858. Moved back, as expected.
Totally unexpected that the shoulder can move forward by a full and proper insertion into the sizing die.
I'm sure I'm not imagining anything, it was a batch of 30, and I measured most of them before and after.
Posted : 02/10/2025 12:57 pm
That is not unexpected. If you measure the shoulder diameter of a fired case and the shoulder of a sized case you would see a reasonable difference. It happens with every bottleneck case to varying degrees.
When your die squeezes the shoulder in towards the centre to reduce the diameter, before it bumps the shoulder, that brass has to go somewhere. It ends up lengthening the case, particularly the shoulder dimension, until the die does the final bump. The bump is what moves the extra brass into the neck.
This is the exact way cases grow. You didn't really think brass flows under a paltry 60kpsi pressure, did you?😀
Posted : 02/10/2025 4:43 pm
Where does the brass go that went into lengthening the case when it gets bumped back again ?
I am who I am - I am not who you want me to be.
Therefore I am me.
Posted : 03/10/2025 2:59 am
Posted : 03/10/2025 4:04 am
This was a technical read, but the second half gave a pretty good explanation why it is a bad idea to have an oily chamber or oiled cases.
Posted : 03/10/2025 7:06 am
Topic starter
Up until this point, I did not explicitly notice that case length increases after full length sizing.
I was looking at the length of fired brass, and trimming before sizing. For the simple reason that fired brass fits over the trim mandrel. If I size first, I need to open the neck a bit to fit over the mandrel.
My first target rifle used 7mm brass necked up to .375, which come out a bit short of trim length. With that brass, after a few firings when the fired case length approached max, I would trim to the minimum length.
I need to measure the case length before and after sizing with that too.
I always measure the sized brass for length before adding powder, and it's not over, so it means that the case length is being pushed out somewhere between the minimum trim length and the maximum case length, maybe one or a few thou, not as much with the BMG brass.
Posted : 03/10/2025 7:57 am
Topic starter
.. but the second half gave a pretty good explanation why it is a bad idea to have an oily chamber or oiled cases.
The British .303 was proof tested using an oiled case. I read that somewhere.
Posted : 03/10/2025 8:00 am
.. but the second half gave a pretty good explanation why it is a bad idea to have an oily chamber or oiled cases.
The British .303 was proof tested using an oiled case. I read that somewhere.
My recollection was that they specified that it must be scrupulously oil free or the test would be invalid. The standards changed over time but that (I think) sticks in my mind from the procedures I rabbit holed in royal ordnance specifications circa 1920-ish. The proof round spec was already well spicy on its own.
Posted : 03/10/2025 8:37 am
.. but the second half gave a pretty good explanation why it is a bad idea to have an oily chamber or oiled cases.
The British .303 was proof tested using an oiled case. I read that somewhere.
There were experiments with oiled cases in machine guns to improve feeding. I do think that they would have beefed up the action to compensate for more bolt thrust.
See comment found in Reddit below :-
"As you may have surmised from the comments, modern machine guns (and modern guns in general) still require lubrication and it's an important enough component of their operation that many people have strong opinions on the subject.
That being said, a few of the weapons you mentioned (the Schwarzlose and the Type 92) require oil to lubricate their cartridges in addition to their internal components. This was done to aid cartridge extraction but at the cost of adding another thing to deal with and providing a convenient avenue for dirt to enter the gun, especially in dusty conditions. As such, it's kind of a clumsy solution to primary extraction issues and as people started to better understand the problem (and patents expired on better mechanisms) the cartridge oilers went away."
Posted : 03/10/2025 9:09 am
Think of the carbon build up and grit trapping potential - oiled cartridges 😮
I am who I am - I am not who you want me to be.
Therefore I am me.
Posted : 04/10/2025 7:11 am
Topic starter
This is the exact way cases grow. You didn't really think brass flows under a paltry 60kpsi pressure, did you?
I don't agree that brass flows on sizing. It flows on firing.
The proof is in rifles with a match chamber and custom neck that don't see a sizing die, the necks are just sized after each firing and reloaded. This brass will need trimming, and if it's never in a sizing die, it means it's flowing in the chamber.
There is not enough force in a handloading press to make brass flow. There is enough to resize a thin brass tube to smaller dimensions.
If you have ever tried to swage pure lead bullets in a handloading press, you'll know how much force is required, and brass can not be swaged.
The case walls expand against the chamber. The die brings the case walls closer together, pushing the mouth out, and the shoulder against the die. It's the same amount of material pressed into a narrower tube, so it comes out longer, but it's the firing in the chamber that stretches it.
We know that brass stretches in a chamber because we occasionally observe the result of repeated stretching as case head separation.
Posted : 05/10/2025 8:01 am