When you do the mission Crossing the Chasm (starter mission, not sure if doable once you reach Haven) you will come across this NPC knight wielding a greyish looking Sentenza. At first, I thought it ought to have been a sentenza. But it wasn't purple, and it looked more similar to the 3* antigua's colors.
Maybe the developers notice how all the piercing guns we have are all slow and don't let you move and they are finally adding this as a piercing antigua in the future?
A lot of NPCs have unique-looking weapons or armor and stuff. Thatd doesnt mean we can get them.
Relatively speaking, a piercing antigua is easy to do, swap damage and make a texture. I think they would have released it by now.