How many firings before growing stops?

[fatelvis]

If you want a accurate depiction of your rifle’s chamber, how many firings on your brass minimum do you allow (without bumping the shoulder)? Or perhaps you do the Whidden method, or some other method to figure headspace in your rifle? Thanks for your input.

 

[Ralph Littlefield]

I can't tell you how many firings your brass will require, but if you are looking to measure your chamber you can just start a SPENT primer in an empty case and let the bolt seat the primer by chambering it. The primer will stand proud of the case and can be used to measure headspace.

 

[Herzo]

If I understand the question, OP is asking if a case will stretch to the maximum headspace dimension after a single firing or does it require several firings to attain that condition.
It is certainly a function of the case construction and the hardness of the material. In order to check the growth of the case, I have machined a bushing that's a slip fit over the neck of the fired case and has a chamfer that corresponds to the angle or the cartridge at the datum point. This allows me to make a relative measurement of the case b4 and after firing. By neck sizing only, you should get your answer upon subsequent firing.

 

[Laurie]

OP is asking if a case will stretch to the maximum headspace dimension after a single firing or does it require several firings to attain that condition.
It is certainly a function of the case construction and the hardness of the material.

Yes and more. Firstly how much 'slack' is there in the new case to chamber fit; secondly the powder charge's fill-ratio and its burning characteristics. In most situations, I'd reckon one or two firings, but it can be more. I realised this recently with a new 6.5mm Grendel Mini Howa and new Lapua brass. My first couple of trial load work-ups used two very different recommended powders under the same bullet - IMR-8208XBR with 100% fill-ratio and at top load slight compression; Ramshot Tac ball type powder with a top charge that only just broke 90% fill. MVs weren't that far apart and using QuickLOAD, neither set should generate any problem pressure levels and should also have been producing reasonably close ranges.

Extraction and bolt opening and case appearance were very different though. The Tac loaded cases had very easy primary and full extraction as expected, but primers were pancake flat for all charge weights. The XBR loaded cases had hard primary extraction for all charge weights, but rounded primers suggesting low to modest pressures (allied to the modest MVs and lowish QuickLOAD forecasts).

Head scratching time! Flat primers plus low to modest pressures usually = excessive headspace. Conversely, the XBR loaded brass showed all the signs of cases that were a longitudinal crush fit in the chamber - very distinct circular marks on case-heads imparted by the bolt turning on a case hard up against it until primary extraction was complete. There were no pressure signs on the case such as excessive expansion in the web area, and as already said very round primers with nice deep and round firing pin strike indentations. So, maybe the two loads had fireformed the brass differently? Decap some cases and get the calipers, comparator and appropriate Hornady 'headspace gauge' out. (A misnomer as it's a base to case shoulder comparator tool, nothing to do with true headspace gauges.) The XBR case readings were 0.005" longer than for the Tac cases. (Eh? !! ) Scratch head a bit more! Measure new cases out of the box - they're a full 0.008-0.010" shorter than the fired XBR readings and around 0.004-0.005" shorter than the Tac cases.

So, we have new brass with a very short base to shoulder length compared to the chamber on a new rifle! (I assume this is deliberate on Lapua's part due to the cartridge's primary use being in semi-auto ARs; maybe a mismatch between US SAAMI and European CIP specs, but that seems very unlikely.) In any event, it means a lot of shoulder movement on the initial firing or firings. The two powder types and their fill-ratios apparently then acted differently in this situation despite MVs and calculated pressures being pretty similar, one (Tac) only doing half the fireform job; the other (XBR) not only doing it fully but ramming the shoulder forward so far and hard, the case remained a tight fit in the chamber.

 

[NorCalMikie]

The case is ALWAYS growing, that's the reason for resizing. It's how you resize that brass that makes the difference.
Standard full length dies (with the neck button) will pull the neck LONGER with every up stroke. Then they need trimming.
Precision shoulder bump dies don't work the same way. I've got BR brass with 20+ firings with NO trimming needed.
On a bolt gun, if you can re chamber the fired brass and close the bolt with no to very little resistance, load it and go.

 

[NorCalMikie]

Flat primers plus low to modest pressures usually = excessive headspace.

And that usually leads to FTF rounds because the case is being pushed forward in the chamber and you're getting soft strikes. THEN () you start throwing $$$$ at it trying to figure out what went wrong and you're chasing your tail. Vicious circle.

 

[Laurie]

And that usually leads to FTF rounds because the case is being pushed forward in the chamber and you're getting soft strikes.

I was pretty surprised that they'd all fired given this excess 'headspace' condition. Looking at my notes, MV spreads were so-so for the XBR loaded cartridges, but pretty good for the Tac examples (by Grendel standards anyway). First firings on the other 50 new cases did see one Viht N530 powered batch of five produce 60 fps ES though which with hindsight might have been light strike induced.

 

[243winxb]

Neck size till the closing of the bolt provides a crush fit. There is no set number. Its all about pressure of the load.

Anyone that started with the Lee Classic (hammer) loader for a bottle neck cartridge learned this .

 

[Bill K]

Neck size till the closing of the bolt provides a crush fit. There is no set number. Its all about pressure of the load.

Spot on.. every case is a product of it's own and no given number will apply. Pressure of the load will effect each.

 

[dgeesaman]

If you want a accurate depiction of your rifle’s chamber, how many firings on your brass minimum do you allow (without bumping the shoulder)? Or perhaps you do the Whidden method, or some other method to figure headspace in your rifle? Thanks for your input.

I just resize without moving the shoulder until I get slightly sticky bolt lift and I set my die shoulder bump to push those cases back .0015.

I like Ralph’s method though - probably only safe using a spent primer - would allow me to “see” shoulder bump.

Another method would be to add layers of scotch tape to a case bottom until the fit gets snug in the chamber.

 

[Ralph Littlefield]

I just resize without moving the shoulder until I get slightly sticky bolt lift and I set my die shoulder bump to push those cases back .0015.

I like Ralph’s method though - probably only safe using a spent primer - would allow me to “see” shoulder bump.

Another method would be to add layers of scotch tape to a case bottom until the fit gets snug in the chamber.

Definitely use a spent primer! Thank you for saying that, I should have said it in my post.