I'm not that far into the game, having recently completed the Horizon mission.  I purchased a pistol damage upgrade from the shop on the Citadel, went back to the tech labs to apply it, only for it to tell me that I needed one more.  Okay.  So I picked up another damage upgrade, this time from Jacob's loyalty mission, went back to the tech labs, and it STILL said I needed one more...

Something isn't right.  I checked the wikia, and it said the Sabot Jacketing upgrade is unlocked after collecting two damage upgrades.
Probably has more to do with game play choices than any tech type matter, so ask in a spoiler / hints forum instead.