this guide says
a +1 weapon counts a magic and can penetrate 5/magic for exampleThere's no way to get natural weapons to break DR/Adamantine or Cold Iron.
an eidolon with amulet of mighty fist +1 can penetrate the damage reduction too
somewhere in the rulebook (creating magic weapons) is written that a +5 weapon can penetrate 5/adamantite so an eidolon with amulet of mighty fist can do it too
right?
p.s.
the ripper (bear form) has 2 reach evolutions. you can have only 1