Work in Progess
Finish your first BANDIDOS mission.
This is your reward.
In the Mission "FUEL THE REVOLUTION" you have to find and kill a target at Vencejo, Isla Santuario.
This target carries the key for the FND Munitions Storage.
Of the 3 camps, it's in the one in the middle.
Enter the bunker and you can climb down the ladder to open the Storage.
The weapon is inside.