Forgive my ignorance guys ! I fired some brand new Lapua 6br brass and case headspace grew .005 but OAL is about the same . How does that work ? Lol
Thx
Mark
Thx
Mark
Headspace grew but OAL did not.
- Thread starter Okumacaptain
- Start date
OAL of the brass alone? Or of the finished round?
OAL of fired case !
jepp2
Gold $$ Contributor
You just blew the shoulder forward. Your case length "may" grow when you size them, depending on how much you set the shoulder back. When I fireform my 20 Bobcat brass and blow the shoulder forward (and change the angle) the OAL is less by several thousandths.
I just figured if shoulder blew forward that the neck would move forward also . Only planning on bumping shoulder .001-.002 .
jepp2
Gold $$ Contributor
Just keep in mind when you FL size. Pic courtesy of Unclenick on TFL forum.
That's really not unusual. Like other's have just said, you've simply moved the shoulder forward. It happens this way: Pressure builds and the neck is first to expand (because of its thinner wall than the rest of the case) and seals the chamber and will not move under the pressure. There's space between the case shoulder and the chamber's shoulder and so the case behind the neck expands filling the void (this is the process for fire forming). Though the pressure has wedged the case surface against the chamber wall, when pressure is released, the case then has some springback. Though you're case headspace grew by .005, it just might grow a little more on the next firing as often the first firing doesn't fully fire form your case to your chamber. The case OAL will grow when you compress the case with FL sizing and the headspace can also change depending on how you've set your die for that.
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What you're seeing is shoulders forming to higher angle, pulling necks back with that.
It's common, and why trimming should not be performed until your cases are formed & stable, and within desired clearance.
Your 'headspace' did not grow.
While case head to shoulder datum increased, your headspace decreased (I assure you).
It's common, and why trimming should not be performed until your cases are formed & stable, and within desired clearance.
Your 'headspace' did not grow.
While case head to shoulder datum increased, your headspace decreased (I assure you).
Guess I said it wrong . I know chamber headspace decreased . I was calling base to datum case headspace .
Brass does not always grow in length every firing. The headspace increased because the shoulder expanded to the chamber.
Just an update ! So after 1st firing my OAL stayed the same but base to datum grew .005 . So picked a +.006 shell holder and it bumped shoulder .001-.0015 and OAL grew about .001 . Does this sound about right ?
Thx
Mark
Thx
Mark
Assuming that your die is screwed against the ram, yes that makes sense. Carry on captain.
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