Soldier of Fortune

This is easiest done by decorating your ship to install a Weapons Workbench, and modifying weapons there; as you'll have access to all the resources stored in your ship's cargo hold - even on the higher difficulty settings you'll want for Reach for the Stars.

Just make a point of applying some kind of modification - basic barrel mods are pretty cheap - to all your weapons as you continue through normal play and this should pop soon enough.

Note the criteria is really about applying 50 weapon mods; not quite the same as modifying 50 different weapons.