Oi , finally unlocked , here is the method i used , hope this will help you .
Delete your savegame , xbox + Cloud. Make sure you have the last version with Patch 19/05/2017.
i started a new game and rush through missions to unlock side missions needed.
As soon as you unlock a sidearm , return to your QG and buy it immediatly before starting another mission.Don't worry you will have enough fund for each.
GRAB THIS ONE as soon as you can
MP-40 Grad located on Village Map 28.11;12.92
MAIN STORYLINE
M1984 Pistol
Herrvalt 99
Garett M9
Wagram 21
SLP .45
SIDE MISSIONS
M1984 Pistol Rail - Side mission Rotki Lions 2
Bull 686 - Side mission Burning Bridge 2
SP M23 - Side mission Loose Ends 3
Achievement unlocked as soon i bought the last pistol.
Really hope it will work for you and sorry for my english