Activision and Infinity Ward released a video explaining the new Gunsmith 2.0 for Call of Duty: Modern Warfare II (and beyond, because Activision said they want the engine to be used universally going forward). Infinity Ward explains how the new Gunsmith works. Gunsmith 2.0 allows you to modify weapons using attachments from that weapon class. Either class, or weapon style. It depends on how the weapon is compatible with the other weapon.