Happened on a raid mission.
There was an Arthron that was completely immun to damage from the assault rifle.
Had to get close and use other weapons.
Second mission same problem.
Arthron takes damage on the legs, but not at the face.
Chiron ist affected too.
At some points Arthron will get 30 armor on most parts. Then the PP AR with 30 damages will be pointless if you can’t aim precisely a part with lower armor, or if you can’t reduce armor before, there’s various tools for that, but also more as campaign progress.
The NJ AR will be more useful much longer, and even later a piercing AR will be better. But in general you’ll often need work on armor decrease aspects.
haha. Thread should be in features/feedback section, not the bugs reporting section. 
But well… thing is that evolution of Arthrons and Tritons happens too quickly, so it may be surprising for new players to encounter immune enemies in first month of in-game time.
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For me it’s more that team get cornered with weapons variations, manage the design and give specificity to weapons types, and builds choices among weapons types.
The PP AR is plain wrong too fast.