BEFORE completing the Gunsmith quests, read the Ghost's commentary on them. He notes that "the last time Banshee made a custom weapon was for Galliya (spelling?) Tuyet- as in Fireteam Tuyet". We should all remember that Fireteam Tuyet's Last Stand was where we went to begin the Pocket Infinity quest. This implies to me that completing all the Gunsmith quests may lead to the biggest and best custom weapon of all.