Help with neck dimensions post-fire

[Subwrx300]

I've noticed something odd while cleaning/prepping my .308 Lapua brass yesterday. Basically the neck dimensions at the mouth and prior to shoulder are significantly different.

Background on cases: Necksized only, 6 fire cycle just completed and annealed after 5th cycle. For these cases (about 100 or so of 300) at mouth, fired cases are measuring avg. .339" OD vs the base of the neck measuring avg.344", a difference of .005" which makes the case look almost crimped even after firing.

In a separate lot of brass (fired more than 12 times with only neck sizing) I get "normal" measurements of .343/4 at all points across the neck. These were fired more recently and had no signs of bulging/weird dimensions.

Picture here with dimensions and clean picture for comparison.

Normal brass on left and "odd" brass on right...

Without markup...

Close up of brass neck with caliper as reference for square:

My initial thoughts were either:

-carbon buildup in chamber at neck region (but most recent fired brass showed no signs of crimping)

-cases elongated into throat on firing (but all cases measured 2.005-2.006 oal post fire)

-annealing inconsistency? But I can't understand how cases would shrink only at mouth.

Any help here would be valuable as I want to be sure that something isn't dramatically wrong with chamber or brass before reloading again.

FWIW, I've been reloading for 7 years and never encountered anything like this before. Google-fu didn't return any good results that would help explain this. Thanks for any input!

 

[Deadshot2]

I've seen this with some of my .308 cases as well but it goes away after a good annealing. I'd make sure that you're getting the necks hot enough when annealing and aren't being too conservative with the "flame time". I anneal my cases by heating to a point just before they start to turn red. About 4 seconds in a Bernz-O-Matic propane torch with a wide open valve. My cases have a definite demarcation line between annealed and non-annealed, much like what new lapua brass looks like fresh from the box.


As for making sure there's no carbon buildup in the neck portion of the chamber I use a .38/.357 brush on the end of a Dewey chamber brush handle (basically a short cleaning rod with no swivel action). I spin the brush in the neck area dry for a few dozen times then use a wet bore mop to swab it out. Repeat a couple times until finally checking with a patch wrapped around the brush (after cleaning it with brake-kleen).

It appears that the mouth area of he necks are a little more work hardened and spring back more than they would after fully annealed.

Anyway, that's what I found with my brass.

 

[Subwrx300]

I've seen this with some of my .308 cases as well but it goes away after a good annealing. I'd make sure that you're getting the necks hot enough when annealing and aren't being too conservative with the "flame time". I anneal my cases by heating to a point just before they start to turn red. About 4 seconds in a Bernz-O-Matic propane torch with a wide open valve. My cases have a definite demarcation line between annealed and non-annealed, much like what new lapua brass looks like fresh from the box.


As for making sure there's no carbon buildup in the neck portion of the chamber I use a .38/.357 brush on the end of a Dewey chamber brush handle (basically a short cleaning rod with no swivel action). I spin the brush in the neck area dry for a few dozen times then use a wet bore mop to swab it out. Repeat a couple times until finally checking with a patch wrapped around the brush (after cleaning it with brake-kleen).

It appears that the mouth area of he necks are a little more work hardened and spring back more than they would after fully annealed.

Anyway, that's what I found with my brass.

Perfect. Thanks Deadshot. I just finished tee-annealing all brass and went to 8 seconds (when the color change rolled over shoulder) to get them more uniform. I think your spot on with the annealing temp not reaching peak at mouth of neck. I'm planning to test this as a followup to my last annealing post regarding accuracy.

I left 20 cases unannealed and will shoot 4 5-shot groups of nonannealed and 4 5-shots annealed, checking for accuracy/consistency and case mouth deformation.