Case head expansion

[BoydAllen]

Die too big for chamber

 

[wvuredneck09]

Why do you think just some of the cases and not others? I just guessed that the die didn't size that far down the case.

 

[BoydAllen]

It is pretty typical that they would not all get tight on the same firing. What you are seeing is the cumulative effect of warm loads when loading with a mismatched die. How far above the top of the extractor groove are you measuring?

 

[Brians356]

I use a blade mic ~at the webline(~.200" forward of groove).

Why do you use a blade mic for measuring the case expansion at the head, vs a regular mic? Or is that only really necessary with the unfired case, before there's any expansion ring to get on top of?

 

[wvuredneck09]

I'll have to check, I measured where the marker rubbed off on the tight cases. Set my case master up so that it indexed to this same location for all of the rounds and checked all of them. Had 4 out of about 20 that measured the same over sized and the rest where fine, 0.004 under the four problem cases. My buddy only had 20 of the recent loads with him, told him to bring the rest over so I could measure them as well. Maybe I'll snap a pic of the rubbed area and the measurement to that point when he does if that will help.

 

[mikecr]

I use a blade mic ~at the webline(~.200" forward of groove).

Why do you use a blade mic for measuring the case expansion at the head, vs a regular mic? Or is that only really necessary with the unfired case, before there's any expansion ring to get on top of?

Blades are just easier for repeatable measure here(spindle contact not turning).

 

[Brians356]

I use a blade mic ~at the webline(~.200" forward of groove).

Why do you use a blade mic for measuring the case expansion at the head, vs a regular mic? Or is that only really necessary with the unfired case, before there's any expansion ring to get on top of?

Blades are just easier for repeatable measure here(spindle contact not turning).

Ah! That's a feature I wasn't aware of, and a very good one. (Obviously I've never had one in hand. But just as obviously it has to work that way.) The turning spindle is indeed annoying. As long as the blade type is as accurate and repeatable, I want one! Thanks.

 

[BoydAllen]

Blade mic's are used to measure diameter at the bottom of the extractor groove. That measurement is all about peak pressure, and relates to things like loosening of primer pockets. Measuring in front of the head is about chamber size and its relation to die size, and is not the proper way to estimate pressure at all. Apples and oranges. The blade mic is used because a standard mic will not fit, and measurements are required to be of a degree of precision that calipers are not capable of.

 

[ericbc7]

With new technology, are strain gauges a practical option? For each rifle the pressure limits are going to be slightly different. How complicated would it be to sensor up a rifle and really know what is going on (pressure wise)?

 

[BoydAllen]

Let's use my benchrest rifle as an example. The case, some of which is occupied by bullet, measures an inch and a half long, from the bolt face. The barrel tenon (threaded part at the back) is a little over an inch, so you can see that there is a problem as far as positioning a strain gauge to read over the case body. For shorter tenons and longer cases, there would seem to be less of a problem. My question would be, why do you need to know? A proper load workup takes care of the issue for me very well. Frankly it is hard for me to take people seriously who are trying to come up with hard numbers for a top pressure load using either a manual or software. This is because chambers and barrels vary significantly in the pressure that I given load would produce. I start low, and do a pressure/node identification, one shot per load test, looking for pressure signs. Never had a problem working that way. I load at the range, and shoot over flags.

 

[mikecr]

This is very much tied to initial chamber clearance, and the barrel steel around your chamber, but none of this tells you actual pressure. You don't measure cases anywhere to determine actual pressure.
Even a strain gauge has to be locally calibrated(with software), just like QuickLoad software.

The 'case head' measure has nothing to do with dies. Dies do not size case heads anyway. Dies/sizing serve only to interfere with this test.
The test tells you the point where growth in/near webs reaches runaway, leading to eventual case head growth(which you cannot undo).
You're not measuring actual permanent change, but instead finding a point where brass will last, or not, for the long run.

While proper sizing certainly increases case life, it doesn't trump what this test will tell you despite.
Put another way, no matter my sizing applied, including custom FL, if I pushed cartridge xxx past web dimension plateau, the pockets would exhibit chronic loosening.
However, I can find that line, which is relatively sharp, with a mic and careful testing.

I have to test for this, like this, because it cannot be predicted otherwise. Not with software or strain gauges, or 'pressure signs' from the bolt, or even if I knew the exact pressures.
The line is important to me because sizing does not restore brass character after yielding. It changes it with each cycle.
Now if the line does not matter, as with a 6PPC competitively loaded, then there is no reason to consider it. In such a case, yielding and short brass life are a necessary given. But for a large LR cartridge keep in mind that 1/2moa is winning performance whether you actually win or not. You can get this with cases that last a lifetime.

 

[BoydAllen]

I have no idea where you got the idea that 6PPCs have short case life. or that most have case head expansion problems. Some shooters discard cases after a match weekend. Others do not, but the reason is generally not that they have been ruined by excessive pressure. Those shooters believe that they get better accuracy in competition starting with fresh brass. If you have a method that is useful to your purposes then that is all of the validation that is necessary.....as long as you are happy with the result. Whether it is my varmint rifles, or a short range benchrest rifle, half inch accuracy is unsatisfactory, something that tells me that I have more work to do.

 

[M14AMU]

Back in the day, I helped a friend, who is a meticulous reloader, with a project, by coming up with the print dimensions for a tight neck .300 Weatherby that had a freebore length that would allow bullets that were seated so that they would fit in the magazine of the Weatherby MK5 action, and touch the rifiling. Because there was no available load data, we approached load workup quite cautiously, starting in the lower loads of the reloading manual, and working up by half grains. I worked with a single case, neck sizing, firing and loading it at the range, with weighed charges. We used a permanent marker to mark the case across the head so that when we measured the belt diameter (with a .0001 mic.) that the measurement would be more uniform. The brass was Weatherby, which I believe was made by Norma. As we started, the first firing produces some belt expansion, even though the fired case gave no other indication of pressure. After that, until the last test, the belt diameter remained stable. As we got to higher and higher pressure, at some point, a circle appeared that showed that the pressure was high enough to move the brass slightly into the ejector hole. The diameter of that rifle's ejector was on the large side. finally, about a grain and a half over the manual maximum load, the belt diameter increased again, slightly, and we stopped. Generally, I do not find head expansion to be the indicator that I want to use for top pressure, because the brass from different manufacturers can be very different in hardness. I prefer to look at primers, bolt lift feel, and in some cases that bright spot that show up on case heads where the ejector hole is. Generally, over the years, I have become less prone to push the limits, having seen examples of what that can produce. Another thing that I keep track of is primer seating feel. If I start to feel primers seating easier, after only a few firings, I know that I need to retune at lower pressure.

+100

Being doing it your way for 50 years and never had a problem!! ;D ;D