You got me. If you measure you case mouth after sizing and its .305, then thats more than enough neck tension. Have you taken apart you dies? I measure .3065"on my expander ball. When it FL sizes without the expander ball, case mouth is .3055". Do you feel a drag when pull you case back thru the expander ball? Also do you have alot of lube in you necks? Don't need that much remember. Could but doubtful be a bad batch of brass they didn't anneal properly. Never seen it but I wouldn't put it past remington.

I mean I have fired 300RUM three times thru the same brass that has been FL sized and still haven't encounter this. Also, when seating you are are not setting up to crimp? Right? Put in a loaded pieced of brass, no bullet of course and put your sizer in until it hits your case mouth and back out one full turn?