Curious case neck/shoulder rupture

[P7M13]

Reloading for my Winchester M70 338 Winmag. Have reloaded many times using Hornady 225gr SPs and RL22, but this is my first time using monos, Nosler 225gr E-Tip and VV N165 powder. I had just come into four pounds of the N165 and wanted to try it.
Loaded a ladder test string starting at 62gr to 75gr. What I've always done : FL size, charge, seat.
Cases were not annealed, as did not see the need. For the ones in this case, these would be their 2nd firing.
First round at 62gr, the neck came off.
2nd through 4th rounds, splitting & cracking at the shoulders.
No other pressure signs.
Number on the case indicates the order the cases were fired. Stopped at #4.
Looking at photo, I had evidently pulled #3 before - probably seated to deeply.

Q: What are potential causes?

Though this has never happened before in my loading, makes me want to take out the cerrosafe and cast the chamber for a sanity check.
The one difference with the gun is I put a walnut stock on and bedded it.
Haven't re-checked torques, although I wouldn't expect that to cause chamber distortion.
Figure at a minimum, pull the rest, dump/deprime and anneal.
Any ideas or observations are appreciated. Thanks.

 

[MagnumManiac]

Have had the same thing happen on brittle brass in the past.
Annealing the brass is the only fix on what's left.
I had them fail during firing and when I sized them…the necks would come off and get stuck on the expander.
The small cracks are because the brass is pulling back faster than the expansion, it has gone too hard.

Cheers.

 

[Frog4aday]

Did you use Brasso to polish your cases? They look brittle like they've been exposed to ammonia.

 

[Seabeeken]

Agreed, brittle. Anneal. Is it winchester brass?

 

[P7M13]

Agree with all that annealing is in order. Will do with all my cases.

Did you use Brasso to polish your cases? They look brittle like they've been exposed to ammonia.

No. I cleaned these up for the photo using 4/0 steel wool.
These would have been the last batch from when I used walnut vibratory tumbling, and had been sitting in a "ready to load" case since ~2014. Have since gone to wet/pins.

Agreed, brittle. Anneal. Is it winchester brass?

Federal

 

[Seabeeken]

Perfect illustration of why annealing is focused at the neck shoulder junction.

 

[PddPdd]

Do you have a tool to measure neck wall thickness?

Only cases 1 & 4 appear to have the shoulder in the same location but perhaps the bases aren't on the same flat surface?

 

[nksmfamjp]

I'm guessing if you look at the rest under 20x magnification, the cracks are already there.