necks too thin?

[savet06]

I have a feeling I know the answer already but I wanted to ask to be sure.
30br .330 neck Robinette Reamer - necks turned to have a loaded round of .328 as best as I am able to measure with my micrometer (.3287 pressure ring on the BIB 118's).
The brass has approximately 20 firings on them and I noticed after this last session that there is some carbon deposit that has crept down onto the shoulder of a few cases.
These necks mic at .0090-.0093 - understand that I am not a machinist and these measurements are a relative mark for my equipment. The other cases that do not have deposits on the shoulder mic at the same thickness (edited after I went back and rechecked my measurements). Loaded round on the deposit case is .327.

Question being is the deposit due to the thin cases or is work hardening involved. Additionally, should these thinner cases be discarded for safety purposes? Have they become thin with use and only a matter of time before they separate?
I have a new blue box of lapua 6br cases at the ready in case these need to be tossed.

Thanks in advance,
Mike

 

[Charles E]

General answers only -- What's safe is a matter of investigating a particular rifle.

No, .003 clearance is not excessive. Some people run that, and even .004.

No, .009 wall thickness is not too thin. With my .261-throated 6ppc, I run .0083 neck wall thickness.

I'd say you should try annealing your brass. If you aren't running (very) high pressures, cases will often last more than 20 reloadings in the .30BR. After annealing, the soot ring should move back to it's normal position.

It has been my limited experience that if your FL sizing is set up right and doesn't push the shoulder back too far, the BR cases give up by the the neck coming off at the neck/shoulder junction. If you're lucky, this happens when you size, not when you shoot. If it happens when you shoot, you'll need to be sure to have a stiff .308 bronze brush in your take-it-to-the bench kit, which will pull it out.

By the way, Robinett, when Randy's name, doesn't have an "e" on the end.

 

[savet06]

Thanks for the reply and the correction on Randy's name.
I had a feeling that it was due to work hardening, but I don't like to assume anything when I am unsure.
This is my first bench gun and the first time I have reached this many reloads on a set of cases. I start getting a little leary when things change with regards to
brass performance.
I will try annealing them and go from there.
Thanks again,
Mike