Can Somebody Help Me On Call Of Duty Modern Warfare 2 For Xbox 360?

Ok so i don’t have online at my current house, and i’m trying to get the attachments for the weapons. Do you have to do the kills online to get the attachments because in offline multiplayer i play my brother and i’ve killed him tons of times with the famas and i still haven’t gotten the attachments. Btw i just got the game for Christmas so any other hints or tips are helpful too. thanks.