I am running into this problem of loose 30BR necks that have me stumped. I have a .330 neck 30BR rifle. After fire forming, I begin with using a .327 bushing, any smaller is too tight. After firing a couple of times, I have to go to a .325 bushing since things begin getting loose. After 2-3 more firings I am at .324 bushing. After one more firing it seems much looser then when I first used it. It seems that at this rate I will have to go to a .323 bushing soon.
I neck turn my brass to .0010. My mic measurement of a loaded cartridge is coming out to .330 even after using a .324 bushing.
What am I doing wrong? Should my necks be thinner?
Any help would be greatly appreciated.