Here is the full list of all 48 Necromunda: Hired Gun achievements.
You have killed 100 enemies with a headshot
Complete the shooting training in Martyr's End
Finish your first secondary mission
Finish Chapter 2
Kill Brickjaw in Koloss 44
Buy an upgrade for your Mastiff
Pet your dog in Martyr's end
Buy your first upgrade to the Rogue Doc
Finish Chapter 1
You have performed 100 Takedowns
Finish Chapter 3
Finish Chapter 4
Finish Chapter 5
Finish Chapter 6
Finish Chapter 7
Finish Chapter 8
Finish Chapter 9
Finish Chapter 10
Finish Chapter 11
Finish Chapter 12
Finish the Main Campaign
Fill one faction bar
Upgrade one implant to the class 4
Wallrun 2500m
Take out 50 shields with the grapnel
Finish a mission with an S rank
Kill 100 enemies while wallrunning
Open 20 chests
Kill an enemy with every weapon
Kill 30 enemies with your Mastiff
Kill 100 enemies by using Augmetics powers
Finish all main missions with a rank of A or more
Find a +3 weapon
Customize every aspect of a single weapon
Amass 100.000 credits
Finish Kaerus without using any other weapons than the Ironfist
Finish Avarus without using the grapnel
Place the 5 bombs in a gap of less than 3 minutes in Escher Lab
Kill 10 enemies in a row with headshots only
Kill a Genestealer with a grenade launcher
Travel 5km using the grapnel
Finish a mission using only special weapons
Kill 5 enemies in one use of Strength Booster
Let Brickjaw live in Koloss 44
Kill 30 enemies with the Blast module
Kill 60 enemies with the Perfect Aim module
Kill 30 enemies with the Crush module
Trigger all mines on Steel way