In multiplayer custom games, there is no black market and no indication of how to unlock the classified weapons. There are no levels in custom games so how do you change your create a class to include more weapons? Thanks
To put it short, in custom games there's no level or black market because you're not in public match. Everything is unlocked except the classified weapons. You can only get the classified weapons if you unlocked them in public match previously. To change the amount of weapons you can use in custom game, navigate to "setup game", "edit game rules", "create a class", and change the "selections allowed" setting to the amount of guns that you want. Please note that you can only bump it up to 17.