This is how you equip locked attachments. Great glitch for people who don't have the time to grind every weapon they want to play with. You can do this multiple times to one gun to create your desired build. To do this with primary weapons you do the same exact steps.
Weapons like the Kar are restricted in CDL, all you have to do is go to custom games and equip it to one of the loadout slots then follow the steps in this video.