If that new ammo has case headspace measuring from 1.623 to 1.64 prior to firing checked with a Hornady headspace comparator gauge, I suspect it's bad ammo. That's assuming you measured correctly. SAAMI specs for .308 Win. case headspace is from 1.627" to 1.634" which is a 7 thousandths inch spread. Your measurements have a 17 thousandths spread.
I, too, wonder how you chambered one with 1.640" case headspace in a chamber that's supposed to be no longer in headspace than 1.634".