In order to unlock this achievement you must find and pick up all 28 collectible weapons in the game, AND all 30 gold bar in the game. I am including the video guide I used, it was extremely helpful. The way this video is set up is it breaks down each area of each mission with text overlay of where the gold bars and weapons are. I recommend pausing the video in each area until you have gotten all of the collectibles available, then moving on to the next area. The video is presented in slideshow fashion, so it is necessary to pause it appropriately. I will also include a list of all of the collectible weapons, and roughly the order in which I got them. This is at your discretion however, as the levels provide you the opportunity, in most cases, to get these in whichever order you like.
Welcome to Bad Company:AEK971
S20K
XM8
870MCS
SCAR
Acta non Verba:M249
Crossing Over:AN94
XM8C
SV98
SVU
MG36
Par for the Course:VSS
NS2000
M24
M16
MG3
PP2000
UMP
PKM
There are no weapons on Air Force OneCrash and Grab:M95
XM8LMG
AKS74u
Ghost Town:AUG
9A91
QJU88
SPAS15
GOL
T194
https://www.youtube.com/watch?feature=player_embedded&v=...Video credit goes to Hiorthen.
I hope this solution helped! Leave a comment letting me know if there is any room for improvement! Thanks.