Here is the full list of all 6 Chambered achievements.
You survived the takeover.
One arena completed. Good work, recruit.
You unlocked everything in the Gun Range! That takes some serious dedication.
You are halfway done with unlocking all attachments and skins!
You three-starred every arena, and unlocked everything in the Gun Range. I honestly didn't expect anyone to get that far... Thank you so much for playing Chambered!
The Chamber doors don't open for another 8 levels. Get back out there!