Here is the full list of all 13 Masked Shooters achievements.
Earn Captain military title (score 400+)
Win a round in Factory level
Win a round in impossible game mode
Win a team battle in a chaos game mode
Win a round battle in 1vs1 - Chaos sniper game mode
Earn Warrant Officer military title (score 160+)
Kill 6 enemies with sniper weapon in City level
Win a round in random weapon game mode
Kill 3 enemies by headshots in a round
Earn First Sergeant military title (score 90+)
Earn Corporal military title (score 40+)
Kill 5 enemies with sniper weapon in a round
Die