In multiplayer, the Ballistic Knife is unlocked at level 15 and the Crossbow at level 33, and they cost 1500 each. Both weapons have no attachments available, but kill instantly if the shot lands on target, though with the Crossbow the player has to wait for the bolt to explode after hitting the target (unless playing in hardcore modes). They also have full mobility, so they do not negatively affect the player's speed.
As for the Death Machine, the Grim Reaper and the Valkyrie Rockets, they are not selectable from the Create-a-class menu, but can be obtained via Care Packages killstreak rewards.
In Zombies, both the Ballistic Knife and the Crossbow are available through Mystery Box, while the Death Machine is obtained as a Power-Up. When Pack-a-Punched, the upgraded Crossbow functions similarly to a Monkey Bomb, distracting zombies to where the arrow lands, and the upgraded Ballistic Knife can revive downed teammates if the player shoots at one (doing this for the first time awards the player with the achievement/trophy See Me, Stab Me, Heal Me). Each Special Weapon can only be obtained by one person at a time in a single game (aside from the Death Machine if the Kassimir Mechanism easter egg and Cryogenic Slumber Party are completed).