Nomad holding the flare gun (in black color instead of orange)
It is a 12 gauge pistol flare gun. Its function is to fire off a flare that could serve as a distress signal.
Ghost Recon Wildlands
The Orion Flare Gun appears as a unlockable skill in the Item skill tree where it can be used to attract enemies. Rather realistically, the flare's bright glare makes friendly thermal and night vision largely unusable if used within it's vicinity.