So that's how you get precursor guns...
 

Other than the one in the precursor system cache, you can get them from killing the ships that Ternie summons with her ability later in the game. I found out by accident after turning some hostile with AOE missiles.

Hehe, someone else figured it out! I already have precursor guns on all my fighters.

Although, I did something different. I would find really tough opponents, set precusor drones on them, than just cloak, sit back, and collect the salvage.

But who has the skill to put precursor guns on your ships?

Ternie.

Ternie gets the precursor technology adaptation skill later on as well (berserk tech is the prerequisite).