OK. I just found out with my Giraud trimmer that my 'headspace' or case head to datum point measurement varies more than I thought. (the Giraud indexes off of the case shoulder to trim and varying trimmed lengths put me on this trail)

The case in question--4 different calibers--3 different types of brass-- are all fireformed, and have been resized, some only once, some 3 or 4 times. I have set up my sizing dies to bump the shoulder back around 3 thou, trying to effectively PFLRS the cases. Using my Stoney Point headspace gauge (case head to datum point measurement) I get variances of .015 from case to case. A lot of the cases in a batch of say 50, will be spot on the same number, but 20 or so will be plus or minus that measurement by anywhere from .005 to .0015 or so.

Is this a tendency of all cases--varying springback or such? My presses are very tight and relatively new, the dies vary in manufacturer, but the results are same. Somewhat interestingly, the cases that vary the absolute least are the 2 WSM's I have, 270 and 300......

Maybe I've just found another criteria to sort cases by, just looking for input.

I know Hotcore, let me have it about the Stoney Point thingy.....but the Giraud and my calipers told me the story to begin with!

Thanks guys--Don

Hey Fish

Thought by now you would have seen this. New cases are undersize, sometimes severely, and they have more springback than cases that have been fired more times. For example measuring on a 30-06 case using the Hornady (used to be Stoney Point) Head & Shoulders Gauge (which I find very useful) and neck sizing only using the Lee Collet I got the following:

New cases - 4.0400" (the measurement doesn't matter, only it's relation to the other measurements)
Once fired - 4.0485"
Twice fired - 4.0500"
3 times fired - 4.0510 (slight crush fit)
4 times fired - 4.0515" (crush fit, fired cases hard to chamber)

As you can see the ability of the brass to spring back is reduced with subsequent firings until it does not spring back enough to prevent a crush fit. After the 4th time I use a Redding Body Die to push the shoulder back to 4.0510" and resize the case body. Partial Full Length Resizing.

So the shoulder is not going to be in the same place everytime until you put it there after PFLR'ing when the cases have grown enough to make it possible.

There is one caveat, you can body size and push the shoulder forward somewhat and then push it back. The thing is I could not push it forward far enough on once fired to create a crush fit so I could then push the shoulder back, it falls about .0015" short. On twice fired I could push the shoulder forward enough by body sizing to get to the PFLR shoulder position.

Make sense?

Bob, that does. I haven't typically measured this enough in previous rifle's reloading's to monitor this. The specific issue I have is that the cases will not be the same Head and Shoulders measurement after firing. It does add up that some of the brass hasn't had the shoulder moved forward far enough to get pushed back equally when PFLRS.

The thing that has me a little befuddled is that if all the cases are plus the measurement of the first case(shoulder as far or farther forward) that the die wouldn't push them back equally, more on some, less on others to end up with a group of cases that are the same after sizing. I am finding that if I have say, 30 cases fired at the range in one rifle, that the post firing positions of the shoulders are not consistent, which I thought they would be, especially as these are full power loads, sometimes in load development, sometimes just shooting a rig I already have and accurate load for. After sizing these case the measurements vary by similar amounts. If I had the same measurement across a batch of cases, it wouldn't bother me, but this ain't the case!

Do you think I should sort the cases by the Stoney Point (Hornady) measurement and keep them segregated?

Even the amount of lube used and the speed of the sizing stoke can vary the datum position slightly.

I pay a lot of attention to getting the lube on uniformly. After the initial sloooow sizing stroke with a dwell at the end I pull the case out of the die enough to rotate (but not enough to touch the expander) and size slowly again. I repeat the rotation at least two times.
My cases come out sized extremely uniform to the datum circle. In addition it helps keep the case head square. My loads datums vary to about the resolution of my calipers ±.001, I wish I had a .0001 reading setup. I was just wondering where I could get a Super mike last night. With a Super mike I could check the technique to ±.0001.

Also possible might be the expander ball dragging the shoulder forward as it pulls up through the case neck.
Woods point of variance in "spring back" of cases is a good one too. This will also affect the the amount of expander pull through case to case.

muck

quote:

Originally posted by Fish30114:

The thing that has me a little befuddled is that if all the cases are plus the measurement of the first case(shoulder as far or farther forward) that the die wouldn't push them back equally, more on some, less on others to end up with a group of cases that are the same after sizing. I am finding that if I have say, 30 cases fired at the range in one rifle, that the post firing positions of the shoulders are not consistent, which I thought they would be, especially as these are full power loads, sometimes in load development, sometimes just shooting a rig I already have and accurate load for. After sizing these case the measurements vary by similar amounts. If I had the same measurement across a batch of cases, it wouldn't bother me, but this ain't the case!

Do you think I should sort the cases by the Stoney Point (Hornady) measurement and keep them segregated?

Don

It may be that you are not actually pushing the shoulder back yet, that you are only starting to size the case body. That would happen on once, twice and 3 times fired cases. Until you have completely encapsulated the case body and shoulder then variations could occur.

Hey, I'm guessing here.

As far as segregating according to shoulder measurements, I never have. Essentially, I'm just trying different combinations of bullets and powders and getting trigger time until I can PFLR on the 3rd or 4th loading.

Used to be I wanted once fired brass in a particular gun but now I want 4th fired brass.

Guys, good intel. I will check a few more cases in order to determine shoulder being moved back specifics.
Bob, 4th fired brass sounds about right now!!!

I've thought about doing a batch without the expander and seeing what the Datum point looks like after that.

Ireload2, interesting technique, I will try that a bit. Try Rutland Tool for a great selection of calipers. They've got several that resolve @ .0001 increments, including digital ones.

Interesting how a trimmer can get you started on a new investigation!