Fireforming

[Hikinghunter]

Howdy. I'm doing load development for my new custom build. When I installed the barrel, I headspaced within .001 of my virgin brass length. As such, I'm not seeing any shoulder growth in my once-fired brass. I assume this makes fire-forming moot for this brass/rifle? Or is the brass still fire-forming to the chamber, just not lengthening?

 

[Tiny Tim]

Alot of times it will take 2,3, or possibly more firings before you have to bump the shoulders back. It's likely still forming to the chamber.

 

[Hikinghunter]

Alot of times it will take 2,3, or possibly more firings before you have to bump the shoulders back. It's likely still forming to the chamber.

Ok, but why would the shoulders move at all if there isn't much room for the brass to expand inside the chamber? If I put a piece of tape on the back of the brass the bolt won't close, so maybe 1thou of headspace.

 

[GSRetrievers]

Shot the same brass without hitting the shoulder sizing, you'll find the answer to your question in the next firing or two.

 

[Mikecr]

Ok, but why would the shoulders move at all if there isn't much room for the brass to expand inside the chamber?

WHY is always a good inquiry.
But I don't know..
I just know it does, and does so evermore with lower shoulder angles.

If this is a barrel nut setup, you may have less growth than shouldered barrels, because the nut pre-pulls ALL threading away from the bolt..
This, engaging more threading at once (think stud connection -vs- bolt).

 

[MagnumManiac]

My 260AI, custom chamber, shows no brass movement for over 6 firings. The chamber has '0' headspace, body has .001" above unfired brass and the neck is the only 'loose' area in the chamber. It has .0025" clearance in the neck on fired brass.
I have not yet reached chamber end on this Lapua brass, and have only trimmed once to square up case mouths, case growth is virtually zero in every aspect.
After 6 firings, I am just kissing the shoulder so the bolt falls with gravity, difficult to measure, but it is around .001" or less of bump.

I am very happy with this outcome, brass will last indefinitely.

Edit to add:
My 6.5x47 is the same as above and was the inspiration for this.

Cheers.