RC 20,after reading your description of how cases fail...I havea different way of seeing it.
I'm not saying you are wrong,I just see it different.
For the purpose of this discussion,lets leave out headspacing on the extractor and out of spec firearms.
To close the bolt easily,we have some head clearance.Not a lot,but some. Agreed?
Some feature,whether it be rim,case mouth,belt,or shoulder,limits the head clearance going forward. The bolt face and lockup limit the rearward head clearance. Still good?
The firing pin whacks the primer.the case makes contact with whatever surface limits forward travel. The "Anvil".
The primer ignites the powder,and pressure within the case rises.
Still agree?
So,with the case forward,where is the thinnest brass,or,where does the brass begin expanding,obturating,to seal on the chamber first?
Its thinnest at the neck and the forward part of the case,like,right behind the shoulder.
And,it gets thicker farther to the rear, nearer the web. Still good?
As the pressure builds,the obturated case actually gets a good grip on the chamber walls. PO Ackley published his observations in "Handbook for Shooters." He took the locking bars out of a 94 Winchester and the brass bturation held the breech closed.
So,the thin walled brass is gripping the chamber wall. The barrel is not stretching. Everything is good . But wait!! We have head clearance and pressure is up there.The case stretches till the case head is supported by the bolt face. The chamber does not stretch,so the forward part of the brass has the friction to stay where it is.
Till you get back to that thick part of the brass,with not so much friction on the chamber,you know,right where the case fails. And that's where the brass case gets pulled like taffy. Its over a short length. Pressure keeps the brass against the chamber wall,and as the case stretches,it thins.Ever do the "Paper clip test"? You feel that stretch ring like an O ring groove.
Cut a case with a stretch ring and look.
It might be that very thin section will crack during resizing,but the stretch ring came during firing,not in the loading press.
And .005 or .007 head clearance can pull a deep stretch ring in 4 or 5 firings.
We do a lot better at .002. Much less stretch. Maybe we'll lose cases to split necks or loose pockets before that darn head separation happens!!
Anyway,thats how I see it.
I have to heartily disagree. The normal push of following the die mgs instruction is what causes the case to crack above the base in short order.
That vastly supersedes any other factors that may or may not come into play in 20 or 30 resizing.
If those other factors are there I suspect the noise level is so low as to be totally irrelevant.
To much headclearance over multiple loadings ( 3 ) is what caused my case failures . I had to much headclearance because I adjusted my dies as instructed by redding and has been the only time I've had case head separations .After learning how to adjust my FL sizing die to size a case to any specific rifle resulting in minimal head clearance I have not had another case head separation . So now I do all the other case prep stuff while minimally sizing my cases ( head to datum ) and have not even come close to another head separation . I conclude that " The normal push of following the die mgs instruction is what causes the case to crack above the base in short order. " which is screwing down the FL sizing die until it makes contact with a/the standard shell holder then an extra 1/4 turn or so . Is by far more likely to cause case failure over multiple loading then ANY other one thing in case prep . Agreed ?
HiBC said:Watch the cartoon and you will see how the stretch happens,
"...I am "bumping" the shoulder back..." No you're not. You're resizing the case back to SAAMI spec. Cases DO NOT have head space. Headspace is a rifle manufacturing tolerance ONLY.
"...headspace on a rimless bottleneck chamber gauge?..." Nothing. There's no bolt face.
And you discovered the important part. If you are reloading for 9 rifles of the same cartridge,the rifles have tolerances. So,you follow the die mfgr;s instructions.
What should we call the dimension from case head to shoulder reference diameter that's often the same as what's labeled on a headspace gauge for its chamber? Any idea?".... headspace on a rimless bottleneck chamber gauge?..." Nothing. There's no bolt face.
Many are convinced the death knell of neck only resizing fired cases for best accuracy started in the 1950's when Ferris Pindell (one of the "P's" in the PPC cartridge family) convinced Sierra Bullets to full length resize their bottleneck cases used to test their products for accuracy. Bumping the fired case shoulder back. 001" or thereabouts made case necks straightest on cases. As rimless bottleneck case shoulders are well centered on the chamber shoulder when fired, the case neck will center well in the chamber neck without touching it. Bullets are therefore well centered to the bore without touching the bore any place.A better alternative is get and use a neck sizing die. The LEE Collet neck sizing dies work great. This leaves the shoulder as fire formed in your chamber. Better option than fiddling with your sizing die.
Avoiding overworking the brass by back and forth full sizing and fireforming when fired again.
Yes I do reload for many rifles of the same cartridge ( 4-308's and 7-223/5.56's and no I never follow those die manufacturers instruction . If I want one load to fire in all the rifles I find the one with the largest/longest chamber and size my cases to fit that chamber . This results in the round chambering in all the rifles .
10 of the 11 rifles mentioned have a headspace in between there respective GO & NOGO gauges meaning sizing the cases for the "long" chamber is not creating to much head clearance for the rest . How ever I do have one 308 that has a headspace that barely passes a field gauge . For that rifle it gets it's own custom sized cases for obvious reasons . If not clear why those get custom sized brass . It's because If I were to size them per die manufacture instructions I'd have something like .015+ of head clearance which may very well create a catastrophic failure on the first firing .
Which almost happened the first time I shot it with factory ammo that had .009 of head clearance . With factory ammo my Head to datum measurement grows .010 from factory new to once fired .
That is a bulge in the once fired factory new Winchester ammo . I'll add those cases weigh something like 25gr less then the Federal cases I also shot that day that did not get the same bulge . My thinking is the Winchester ammo has much thinner walls then the Federal cases do .
In closing , no I never adjust my FL sizing dies using the manufacturers instructions . IMO they really should stop using that exact wording . They should explain "why" they recommend that and how it's likely not going to be best for most applications . When I FL size a case per Reddings instructions using there standard FL sizing die and standard shell holder My cases come out .005 shorter from head to datum then my Forster GO gauge . Keeping in mind this is about reloading and that there is only one part of the cartridge that is reloadable . So the die manufacture should keep that in mind for there customers . Very few of us only want one or two loading before we have to scrap are brass . I know I was upset when I learned the instructions are why I only got 3 loadings from that brass and the 200+ pieces left were scrap now because I followed there instructions .
I'm a lot older than I used to be and my eye's aren't as good. But that does not look like a bulge to me. Look's like where the die quit sizing the case.