Here is the full list of all 12 Warhammer 40,000: Boltgun achievements.
Find all Secrets in Chapter 3
Find all Secrets in Chapter 2
Complete all Missions on Emperor’s Mercy Difficulty
Complete all Missions on Exterminatus Difficulty
Complete all Chapter III Missions
Complete all Missions on The Emperor Protects Difficulty
Complete all Missions on My Armour is Contempt Difficulty
Find all Secrets in Chapter 1
Complete all Chapter II Missions
Defeat a Great Unclean One
Complete all Chapter I Missions
Defeat a Lord of Change