Marathon: Durandal
15 Achievements
250
0-2h
Xbox 360
Xbox One
Xbox Series
Bigger Guns Nearby
Got the Assault Rifle.
14
How to unlock the Bigger Guns Nearby achievement in Marathon: Durandal - Definitive Guide
EYEHUNTER 42
TrueAchievements
TrueAchievements
This can be found on the first level. It's to your left of the start. See clip for more details and remember to check your map with 
Pyroman31, baylxr, Andrew, x360a
Follow the walkthrough HERE for the first level and it will lead you to the assault rifle.