Modern Warfare 4 MP has Apex Attachments.
Every gun has 1 Apex Attachment you get when you reach max level. This attachment doesn't take up 1 of 5 Gunsmith slots.
The attachment does unique changes to weapons.
Examples: 1911 pistol gets an underbarrel shotgun. The M4 changes to SMG-style. Sniper gets a throwing knife equipped underbarrel.