Since The Division 2's huge Tidal Basin update, players are finding ammo for signature weapons—weapons specific to each of the three end-game specializations—hard to come by. It appears to be a bug, and Ubisoft has confirmed it's investigating the matter.
Numerous reports on Twitter and Reddit have highlighted the problem, which appears to affect all three specializations. Some players say they haven't found any signature weapon ammo drops in hours of play, while others are saying they're finding much less than usual. Ubisoft is "looking into" the problem.
The team is looking into the Signature ammo drops. Thanks for reporting!April 6, 2019
As a workaround, you can get some ammo by switching specializations at The White House, and then swapping back, which should refill your signature weapon. It will give you some ammo for the fight ahead, but it's hardly ideal.
While you wait for Ubisoft to fix the problem, check out our The Division 2 specializations guide, which goes in-depth on each of the three skill trees.