the whole neck tension, diameter thing. i know really, only what i have read in reloading manuals. that the i.d. of the neck should be .001" smaller than the actual bullet size. OK, fine, i get that. but there has to be a lot more to it for precision loading. other wise, there would not be a need for bushings and buttons, and all of the other neat little devices to infinitely change the sizes and tension on those bullets. i would think that the goal is for each neck to have the exact same tension, on every bullet. i understand how turning the o.d. of the neck helps even out that tension. is there a way to actually measure the tension itself on any given cartridge neck? //where i am at: i have been on an accuracy kick for the last 2 years now + or - a few months. i have gotten all of my production rifles to, or under moa loads for each and every one, with standard load equipment. on my best gun, i am nearing 1/2 moa. i was thinking about this neck tension thing, so i got out my dies, and measured all of the expanders. on all 4 of the .30 caliber ones, i got different readings (.3064-.3072"). i know all the bullets read within .0002" of .308, so obviously, i have an issue. plus i think the brass will spring back some after the expander comes thru. my brother is a machinist, and i load for 2 of his 30 caliber rifles, so this will be how i get him to help me make, modify or get what we need. it would be a lot easier if he did not live 215 miles away however.