I see.
I found a decent thread that explains why it happens, sort of. I understand that when FL sizing it moves the brass up into the neck. But why wouldn't it just flatten out during normal sizing/firing operation?
A couple of questions I couldn't find answers to in searching.
1. Is the donut only formed in certain case types or certain methods of sizing?
2. Do donuts commonly occur in various saami cartridges where the correct headstamp brass is used (not necking up or down)?
3. Is the dreaded donut likely to be a problem for me in this cartridge scenario using the same sizing procedure I always have just because it's a 284win case necked up to 30? (FL sizing by 0.002 shoulder bump each firing and annealing every 3 firings)
My method of prepping this 284 brass was the following:
1. Used Lyman M 308 expander die to expand necks
2. FL sized the cases with a 30-284 rcbs die set to where it bumped the shoulder 0.001 or less. I left the expander ball in the die. I always FL size virgin brass to make sure they're all uniform.
I haven't verified that my brass has a donut. But, I couldn't see one on the outside of the necks and it felt normal when seating the bullets (bullet base went below neck/shoulder junction).
I am now worried that I'll have to start neck turning. I really don't want to add another step to brass prep, I don't have a neck reaming or turning tool, and my wallet is empty!