Cases shrunk with first firing?!

[jonr]

Just fired a six shot group of .308 at 100 yards. Grouping was GREAT!
7/8" overall. Three shots were touching. One "double holer".

42.5 grains Varget, Sierra MatchKing 168 grn

My question is:

These were new, unfired Winchester cases. On average they are 2.004 - 2.007 new. They were neck sized only with a Lee Collet die.
Now after firing they are 2.001-2.004. I did not measure these exact cases before firing but the bag is pretty uniform.

Why would they shrink with firing, I thought cases grew longer with firing?

Thanks!

Jonr

[jgalar]

The new cases are undersized in comparison to the chamber. When they expand to fill the chamber the brass must come from someplace. If you neck size from now on the brass will get longer.