All Adverse barrels on the FiNN completely change the gun, lowering its damage per shot but very drastically increasing its rate of fire - to levels unmatched by almost every other gun in Warzone. The time-to-kill is far better with an Adverse barrel than without.

Unlocking the Finn will be easiest if you actually own the full Modern Warfare game and can play the traditional multiplayer mode, but this can be done in Warzone as well. If you have access, load up the Shoot the Ship game mode that is currently in the rotation.