To unlock the sledgehammer in Black Ops Cold War, you'll have to become a master of melee combat. The task you'll have to complete is simple enough in theory, but requires quick thinking and a not insignificant amount of luck to pull off. Here's what you'll need to do.
How to Unlock Sledgehammer in Black Ops Cold War
The in-game requirement for adding the sledgehammer to your arsenal is as follows:
"Using the Combat Knife, kill two or more players rapidly using the combat knife in 15 different matches."
Double kills aren't so uncommon in Black Ops Cold War, but to get one using the knife requires a special kind of chaos. Enemies have to either fail to see you coming, or they have to be taken so off-guard that they fail to shoot you at range, before you even get close enough to slice them up.
If you do succeed in completing this challenge, you'll see the sledgehammer in your inventory, in the melee section. You'll also receive 1,000XP for your trouble.