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.
Based on the real-world Stoner 96, the FiNN LMG fires 5.56 ammunition at a relatively low rate, making it a very controllable alternative compared to other LMGs in Modern Warfare and Warzone that fire larger bullets much more quickly.