The color change towards the necks is annealing. Mil-spec production requires that the annealing color change *not* be polished out.
If I was concerned, I would pick the one with the most ring marks towards the base, pull the projectile, remove the powder
and probe above the web interior with an angled pick. That will permit detection of the stretch line which is the signature of the incipient case head separation.
You could also fire the primed case and then vertically section the rear of the case to look for that line too.
http://www.thehighroad.org/archive/index.php/t-734058.html
If I was concerned, I would pick the one with the most ring marks towards the base, pull the projectile, remove the powder
and probe above the web interior with an angled pick. That will permit detection of the stretch line which is the signature of the incipient case head separation.
You could also fire the primed case and then vertically section the rear of the case to look for that line too.
http://www.thehighroad.org/archive/index.php/t-734058.html