Case length shrinking?

[Greasegun]

Never trimmed my lapua 6.5 creedmoor cases, as they measured about 1.912-1.914 after the first firing. (I didnt buy virgin brass, used brass from Berger factory ammunition) After the second firing they are at 1.908-1.913. I dont remember measuring any that short after the first firing. FL sized them with a bushing FL die after first firing. Have not FL sized yet after 2nd firing, these lengths are on fired unsized cases.

Has anyone else experienced this? In the past I had only seen brass grow in length. I hope they dont get much shorter, as I understand a carbon ring can form in the chamber neck area void that isnt occupied by the case neck.

 

[MagMan]

What is the body diameter doing. The only way they could get shorter is if the body grows or the shoulder angle changes.

 

[Greasegun]

What is the body diameter doing. The only way they could get shorter is if the body grows or the shoulder angle changes.

The body mics exactly .002" larger behind the shoulder after firing. FL sizing pushes that back down to same as before.

The die used is an L.E. Wilson FL bushing die. Their bushings by design have .002" taper in them. The mouth end of the neck will measure smaller than the back end of the neck after sizing. I wonder if that is forcing the necks to shorten when sizing. Ill have to check some before and after sizing to see if thats it.

 

[MNbogboy]

Sounds normal the second firing allowed for more growth than the first.
They will lengthen after they are FL sized providing you don't bump them more than .002 - .003 chamber length.

 

[Greasegun]

Sounds normal the second firing allowed for more growth than the first.
They will lengthen after they are FL sized providing you don't bump them more than .002 - .003 chamber length.

Sounds logical. Ill check again after I size them. Next question is, will trimming them all to the shortest length really help anything if they are all with .005" min to max?

 

[MNbogboy]

Sounds logical. Ill check again after I size them. Next question is, will trimming them all to the shortest length really help anything if they are all with .005" min to max?

Personally If there are only a small percentage short (.005 or less) I would trim to the max. Short cases have been correlated with carbon rings and short necks may not "seal" as well/fast as full length necks. The second and third firings will probably equalize the short ones for trimming with the rest.

 

[Greasegun]

Personally If there are only a small percentage short (.005 or less) I would trim to the max. Short cases have been correlated with carbon rings and short necks may not "seal" as well/fast as full length necks. The second and third firings will probably equalize the short ones for trimming with the rest.

Yeah unfortunately they are all toward the short end. Thats why I never trimmed them. Would love for them to all be the same but then they would all be at the min spec so dont want that either for the reasons you stated

 

[Greasegun]

Sounds normal the second firing allowed for more growth than the first.
They will lengthen after they are FL sized providing you don't bump them more than .002 - .003 chamber length.

You were correct. After sizing they measure about .002 or .003 thou longer than before sizing...so they are not shrinking. Not really getting any longer either.

I made a plug to measure the neck length in my chamber. Its a light press fit in the neck, which I shortened to accomadate this plug. I chambered that case and removed it. It measures 1.930 long. I can see Ill probably never need to trim this brass unless I decide I want them all exactly the same. If I were to do that now, I would have to take them all down to 1.911, which is pretty short. I wonder if the carbon ring is a problem if I clean my bore after every range session?