6.5 Creedmoor - Velocity higher in Virgin Lapua brass than subsequent firings

[Timberstalker]

Has anyone out there seen a difference in velocity using the same power charge in the 1st firing of Lapua brass than subsequent firings? I neck sized before firing virgin Lapua and before the 2nd firing with the same .289 bushing.
!st firing 41.6gr H4350 (130gr VLD) -> 2709
2nd firing 42.5 > 2706
Both produced practically the same group size, just a different shape hole.

 

[Jrb572]

Did you only neck size your brass or FL size?

 

[rijndael]

What's the wet/water capacity differences?

 

[Timberstalker]

I only necksize until they start to show resistance in chambering. Then I FL size.
I have not checked the water capacity differences, but virgin will surely be less, since the shoulder to base (headspace) is .004 less on the virgin.
On a thread I read recently someone proposed that shorter headspace results in lower velocity because some of the charge's energy goes to inflating the brass to the chamber dimension.
Another theory is that shorter headspace creates more pressure earlier in the ignition due to less cabin space.
Another person pointed out that it is varying chamber dimension, not shoulder differences that makes a difference since brass is soft and inflates easily.

As a tag on to my original posting, the 41.6 charge that originally produced 2708 did drop down to 2670.

Just wondering if others have actually seen a difference over a chronograph like I did. I have now loaded the same brass for the 3rd time, and have some virgin to load for a direct comparison loaded in the same session, so I'll check it out later this week to verify.

 

[Niles Coyote]

I have noted a change from first fire to subsequent firings on all brands of brass... BUT, IIRC it was a slower first firing... and around 15fps.

 

[Deleted member 10043]

What chronograph are you using?

 

[918v]

Your chrono is defective