Once fired brass is no where near chamber length....do i need to trim at all?

[gidds]

Have had some success with OCW and reloading for 308 in general, but I am trying to clean up my process in hopes of much better consistency.

I have about 50 or so once fired Lapua brass cases from a 100 round lot. I measured the following....

Virgin Lapua brass is measuring 2.006 to 2.0065 overall length with calipers.
My once fired is measuring 2.006 to 2.012 (a sample group averages around 2.006 and another sample group averages around 2.012) No idea why the difference.

When using a short case and the Sinclair chamber measuring insert (plug that goes in end of the case and compresses to length of chamber) I get a consistent chamber length measurement of 2.055.

So it seems that my brass is no where near the length of my chamber.

So my questions are:

Is this typical?
Do I need to trim at all (even to make uniform)?
Will this condition greatly effect accuracy?

Any other recommendations are appreciated.

 

[eklarsen]

Do you need to trim... No. Should you trim them all uniform... Yes.
The reason to reload is so that you can make more accurate ammunition, the way you do that is eliminating all the variables that can come from factory ammunition, keeping all your brass the same length takes that issue out of the equation if you have accuracy\performance issues. I dont know the .308 well, but I think the 2.006 or just slightly longer is ideal so long as it is below 2.015 since your virgin brass is 2.0065 use that as your trim to length then all will be uniform.

 

[gidds]

Thanks eklarsen, and I understand your response. I should trim to make them all the same.

Let me expand on my concern above:

-I was thinking that the closer the brass gets in length in relationship to the chamber, the more accurate it would be (as long as it chambers and with no over pressure signs)
-Will repeated firings cause the brass to ever lengthen from 2.012 to 2.055 (if not trimmed every time)

I guess this is more of a conceptual question than a technical question......trying to wrap my head around it.

 

[XLR308]

Before you just start trimming have you measured your brass from base to datum of the shoulder?
You did a work up from min to max so your virgin brass saw pressures that steadily increased that would be the most logical reason for the variation I can think of.
If you kept them in firing order you could verify my assumption or call bullshit.

 

[FLIGHT762]

Yes, it is typical that actual chamber length is well above the SAAMI maximum listed length. Two of my 308 Win. chambers measure 2.040" and 2.045". SAAMI maximum is listed @ 2.015". You actually don'y have to trim for some time.

 

[eklarsen]

If it was me I would stick with the SAMMI spec and not worry about the .03 additional in that chamber, that will allow you to continue firing even if you accumulate some carbon or dirt. Worry about the uniformness of the brass and the seating depth (ogive to land jump) find the most accurate and then keep it consistent.

 

[gidds]

Before you just start trimming have you measured your brass from base to datum of the shoulder?
You did a work up from min to max so your virgin brass saw pressures that steadily increased that would be the most logical reason for the variation I can think of.
If you kept them in firing order you could verify my assumption or call bullshit.

I hope I still have some load markings on the cases (sharpie).........I think I may have tumbled them for a bit, but I understand what you are saying. With that being said, I did measure from the base to the datum (using FA Digital Calipers, Hornady insert holder and Sinclair 20 degree bump gauge insert) and got the following measurements:
Virgin brass 1.564 to 1.565 average
Once fired 1.569 (very consistent)

The way I understand the thinking is that I need to neck size them back .003 (from 1.569 to 1.566).

 

[FLIGHT762]

[QUOTE="gidds, post: 7014609, member: 16829"
The way I understand the thinking is that I need to neck size them back .003 (from 1.569 to 1.566).[/QUOTE]

WTF are you talking about "neck sizing" them back .003"? Neck sizing has NOTHING to do with shoulder bump. Neck sizing doesn't bump the shoulder back. You F/L size them, adjusting your F/L die to (bump the shoulder back) .003" or LESS.

 

[Beneviolence]

Your variance in once fired may come from various stretching of the brass or primer cratering/seating depth from different pressures during load development. My experience is that the brass actually shrinks the first 2-3 firings if you’re neck sizing because the expansion of the case diameter to fill the chamber causes a shortening in length.

 

[supercorndogs]

Are you de-capping before measuring? Often a little primer crater or protrusion will throw you all over the place if you don't.