Collecting all the weapons in the game is no easy feat as some of them appear few times or even once, therefore a without proper planning failure becomes unavoidable. The difficulty come from the rarity of some guns combined with the fact that you need to get the correct weapon almost every level

This guide will enable you to get every weapon in ONE playthrough by highlighting the required weapons to extract in such fashion that the you will earn the achievement before the last mission.

Weapons can be split in multiple categories:

• stock - weapons already available in your arsenals (Mini Uzi, P89 etc) when you meet Steve at the safehouse. 5 primary and 4 pistols
  
• buyable - you can spend cash as an alternative to exfiltration (useful if you mess something up)
  
• unique - only 2 weapons that can't be bought and only one instance of them exists
  
• rare - weapons so scarce that only a few appear and not on many levels, 6 of them, but 2 can be bought

Below is a table with weapons appearance on levels. Apartments levels was excluded. On Office building you can return to the bottom floor, but on Police Precinct level you will not return to the first part (Road to ...). On Steve level of Epilogue (Floor 6) the level ends with the last kill, so plan accordingly.



A google sheets with checkboxes and rules is available for your convenience, just access the link and make a copy:
https://docs.google.com/spreadsheets/d/1tR6War7UXXO3TN_qUIw3mXRAUDnm5t0J6L7-ePYnTHk/edit?usp=sharing