After progressing through the game, you will meet with Leonardo da Vinci. When talking with him, he’ll tell you that he has created several war machines for the Borgia against his will. To access these side-missions, you must first kill an Overseer, and obtain the map to where its located. After obtaining the map, start the mission. The missions are very linear, and can be completed within 10-20 minutes. Complete the mission and the achievement will unlock.
Phase 1: Kill the Templar Overseer to recover a map
Phase 2: Burn the blueprints (remain undetected)
Phase 3: Trigger the explosives
Phase 4: Capture the Machine gun, escape with it, and destroy it.