There are two versions of weapon in the Fallen Ghosts. One is available at the beginning but only has a limited sum of attacments, things like grenade luncher and many scopes are not available for R4 and MDR. The other kind need to unlock through the game progress but can be modded normally with those usual staff that it should has. But the problem is, the waepons added in the main game is the first kind, the limited one. Firstly, i dont understand why the limited one was added instead of the normal one. Then, i dont really see any point in adding a "limited-but-useable-at-the-beginning" version of weapons in the Fallen Ghosts.