Resourceful Achievement

Found weapons are any weapon that is found within each mission, either from caches, lying on a crate or looted from dead enemies. Since they have limited ammo, simply pick it up and get as many kills with it as possible before it runs out of ammo. Repeat this process until the achievement unlocks.
 
Refer to the ‘Mop Up’ section in the roadmap for a farming method to make this go a lot quicker.
 
To check your progress, press then navigate to Service Record > Medals > Weapons and Items and look in the first section to see your medal progress.