Captured outposts often contain weapon crates. Focus on clearing the map icons before doing the main missions.
The gold standard for a secondary weapon when you are infiltrated deep in enemy territory. A Note on Fair Play sniper ghost warrior 3 unlock all weapons cheat
The safest way to "cheat" your way to a full arsenal on PC is to bypass the progression system entirely by using someone else's progress. Captured outposts often contain weapon crates