I have this issue with 3 weapons. I play on steam and have tried all suggestions and none work.
The guns in question are the KTS100 MK8, the SL9 and the SG553R. all 3 show the challenge is complete but the weapon is still locked and unequipable. I also have an issue with the Rail Cover attachment from the assignment for the rail cover attachment. The assignment shows complete but the attachment still says locked in the gunsmith menu (despite the attachment spot clearly being unlocked). it may be important to note the player card title "Compact Fire" unlocked through the assignment "CLOSE QUARTERS 2" (the assignment for the SL9) is also locked so it appears none of the rewards of these assignments are being granted.
