User blog comment:KATANAGOD/Leaked First-Person Gameplay Screenshot from Black Ops II/@comment-1726879-20120503111132/@comment-4025623-20120503203420

Well, any XM9 is very unlikely to be a derivate of the XM8, because any major addition causing a new designation would probably happen after the time if it was ever adopted. The X just means experimental, and would be dropped immediately upon full-on adoption/introduction. And then any new designation would probably come in the form of an M8A1, A2 etc. If you mean it being an "XM9" for the game, then yeah could be, it could be whatever Treyarch wanted it to be.