Sizing 300 AAC

[thl]

Rifle is Noveske N4, dillon 550 press with Lee ? dies set to slightly cam over. Fired cases measure around 1.0830 from case head to datum line ( lnl headspace gauge insert . Factory ammo I have measured is about 1.0650. I was sizing my brass down to about 1.0720 which chambered fine ( rifle was clean) but the shoulder would get bumped back to 1.0685 consistently just by chambering.

Do I size my cases based on data from fired rounds or on the shoulder bump I get from chambering and why is there such a big discrepancy? Maybe Bolt face getting pushed back that far until the lugs lock up completely with barrel extension when gun is fired? So far I haven't had evidence of either case head separation or had to knock out a stuck round but I haven't got it truly filthy yet either.

Edited April 8, 2014 by thl

[ltdmstr]

How is the shoulder being bumped back to 1.0685 when chambering if fired rounds measure 1.0830? That doesn't seem to make sense. If you use a bump gage to measure fired brass then move it back a couple of thousandths it should chamber without problem.

[Tom Freeman]

Resize the brass until the bolt closes on a loaded round and still extracts easily. Dont worry about the numbers. Just bump the shoulder back a couple thou and load.