Its not modded but you can only get it through a save on a computer transferred into your xbox
1st step is denial broseph
This gun cannot be obtained by legitimate means. The Error Message can only be obtained through third party programs or by trading with someone who hacked it into their game.
aka hacking/modding the game save.
TL;DR: it is an NPC Weapon that can only be obtain through modding (hence why it is known as a modded weapon -- yet not exactly in the same class of the "black titled" weapons) --- XBL GT/PSN: Tsuruke.