In the campaign, the MK18 carbine is featured twice as Preacher's primary weapon, in the first mission, Unintended Consequences" with a Trijicon SRS02 reflex sight, an IR light that cannot be used, and a Suppressor, and last mission, "Shut it Down", with a Trijicon TA31ECOS with a secondary RMR red dot sight, the same IR light, and a PEQ-15 IR light/laser combo that can be seen when the night vision is used. This optic is actually unavailable for use on the weapon in multiplayer (and would be useless, anyway, due to MK18's short effective range).
Featuring excessively low recoil, the DD MK18 is a decent weapon for those who don't care about range (quite low and unable to be greatly improved) or stopping power of the single shot. It also comes with a generous ammo load of 181 bullets total. Boasting a high rate of fire, the DD MK18 makes quick work of any enemies from close range, especially with the help of the Signal Scan. The low recoil and high rate of fire also allows for kills at a longer range, although it is still outclassed at range by many other weapons.
With a moderate reloading time, the DD MK18 can be considered average in this class of weapons. It has a higher damage profile than the HK MP7, yet it has 10 less bullets per each magazine, and runs out of ammo only slighty slower. So be careful not to miss your target, or you might get killed while reloading.