THIS ACHIEVEMENT IS NO LONGER AVAILABLE
AS THE SERVER HAS BEEN SHUT DOWN.
This secret achievement is,
Dist. Service - Destroy unidentified enemy weapon
There are several conditions that must be met for a unidentified weapon to become available to fight.
Every man and his dog has their own theory on the exact conditions and fierce debate often erupts when people list the particular conditions they believe are required, so I'm not going to fall into that trap.
You can visit the Chromies forums for that.
However below I listed some (what I believe to be) facts to keep in mind regarding the secret weapon.
The Sal Kar weapon is only available for combat at Gazi's South Cemo Oil Field.
The Morskoj weapon is only available for combat at Xivera's East Salma Woods.
The Tarakia weapon is only available for combat at Baleares' Wakool.
Most importantly each weapon can only be brought out once per war.
Good Luck, and I hope you don't loose as much sleep as I did trying to get this achievement.
For more info check out the fantastic site http://www.chromehounds.addicted-gamer.com/?show_faqs=sw