These two weapons are two of the guns that get the most benefit from the attachment, as it changes the guns from a two shot kill to a one shot headshot. This is achieved thanks to the unique increase both weapons gain, a 1.8 headshot multiplier (if they got a 1.5 multiplier, the attachment would make no difference).
It is a more effective attachment for Burst Fire and high-recoil weapons. Burst fire weapons greatly benefit if any of the three or four rounds hit the head of the opponent, and high-recoil weapons will direct their rounds upwards and towards the head, meaning the last of the shots fired are more likely to guarantee a kill.
High Caliber returns in Call of Duty: Black Ops 4, where it raises the headshot damage multiplier to 1.5x as seen in past games. High Caliber is available on most assault rifles, sniper rifles, and pistols. Equipping High Caliber also gives the player the option to equip a variant known as High Caliber II, which increases damage to the upper chest, and to the torso on sniper rifles only.