The only way to accurately measure neck wall thickness is with a ball micrometer or similar. the only dimensions you need to be concerned about are sized neck dia. VS. bullet seated neck diameter. As the other guys say .001" to .0015" for target ammo, .003" to .0035" hunting. Keep in mind that varying neck tension can have an effect on accuracy. Yours might actually shoot better at .002" :) ..... If you jam your bullets into the lands .002" neck tension, for me, is minimum to prevent bullet set back when chambering a round. My 6.5X47L measures thus: .290" (bullet seated dia.) - .264" (bullet dia.) = .026" divided by 2 = .013" wall thickness. Apparently the .260 has thicker necks........ Or maybe you formed from .308 brass??