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