I'm pretty sure this is just a bug or miscount. The game says I am missing a weapon attachment which is preventing me from having a 100% score on the campaign screen. I have beat the game several times, and I have unlocked every attachment. Correct me if I'm wrong... There are 13 total attachments, but only 12 weapon attachments are in the campaign. The 13th is the Halogram attachment that is only available in multi-player.

Does anyone have 100% campaign score?