The Xent tech light rifle prototype is a reward of a non repeatable lengthy mission, and is used to make a nice weapon called Zorra HK.
http://cyrenesecrets.com/how-to-make-zorra-hk-pkhunting-goodie/
(number of required Lazidol parts has been changed: you need 5 of each now)
Lazidol parts are not easy to obtain... not easy at all. So many prefer to sell the Xent tech light rifle prototype when they finish the mission.