https://www.gamespot.com/articles/call-of-duty-black-ops-4-new-signature-weapons-rev/1100-6461465/

Activision has announced Call of Duty: Black Ops 4 will introduce cosmetic variations of the base firearms called signature weapons. Two of the signature weapons can be unlocked before Black Ops 4 even releases, but you'll need to complete in-game challenges to get the rest.

Signature weapons do not provide any in-game benefits over the base weapons. Instead, they act as bragging rights. Most signatures are locked behind specific challenges that will test your skill in different ways. Completing different challenges nets you additional rewards, with the Mastercraft variant being the final and most difficult design for all signature weapons. If you run into another player with a Mastercraft version of a signature weapon, you'll have a better idea of just how skilled they are and how much time they've invested into Black Ops 4.