308 Case Length - Puzzling

[constable79]

Looking for advice/education from the experts. I have been reloading for 25 years but this is my first time reloading for .308. I have loaded tens of thousands of .223 but have not noticed this issue.

Background

Rifle: Rock River Arms LAR-8 X1, 18" barrel

Brass: once fired PMC commercial, once fired Lake City NATO. Trimmed to 2.005"

Bullets: 168gr SMK's

Powder: RL15, 43grns

COAL: 2.800

Brass was full length resized with the shoulder bumped back .004 from cases fired in this rifle. Sizing was confirmed using case comparator and a case gage. COAL was confirmed using calipers and COAL comparator.

Cases were sized on a Rock Chucker single stage and the the cartridges were loaded on a Dillon 550.

Since this was my first time reloading .308 I took some measurements of the fired cases before and after sizing. I discovered after firing the cases they hardly grew at all. After sizing the cases grew approximately .005".

The question:

When I measured the cases before sizing I found some of the case lengths to be in the 1.998" range which is considerably shorter than the 2.005 length they were trimmed to. I did not mark my reloads so I could have picked up someone else's brass with mine although big coincidence the headstamp was the same. This is the first time I have ever measured a fired case prior to sizing. I didnt think the brass would shrink since it headspaces on the shoulder and not the case mouth. I'm looking for someone to educate me or explain how my brass could have shortened during firing and why it was only some of the cases.

I apologize for the long post but wanted to give as much information as possible.

Thank you in advance for sharing your knowledge and experience.

[Steve RA]

I don't remember seeing any shorten on firing, they will lengthen slightly when sized as they have become larger in diameter when fired and then when sized it will make them slightly longer as the brass was slightly stretched when expanding to fit your chamber.

I'd check again with brass you are certain came out of your rifle.

[Matt in TN]

I have found cases shorter than trim length after firing in my FAL. I thought it odd too, but never followed up on it. Curious to hear more -

[Matt in TN]

Thinking more on it - if the brass expands in diameter to fill the chamber, it makes sense that the length would shrink. That brass has to come from somewhere.

Then when you size it down you reduce the diameter, and the brass grows in length. The case wall is usually thinner after firing, so the length is longer than your trim length because you have extra brass from the case wall.

Edited July 6, 2015 by Matt in TN

[Steve RA]

That is why you will eventually get case separations if you reuse it enough. You can check by taking a piece of wire and putting a 90 degree bend on it about 1/8" from the end and run it up and down on inside of case near the base. If you feel a depression that case is getting there.

[constable79]

Thanks for the replies guys. Appreciate your thoughts.

[dauntedfuture]

It is often the case that the OAL of a case will shrink a little after you shoot it and grow after you size it. Im sure you are trimming after you FL size, correct.

[constable79]

Yes, I trim after full length sizing