Add Destructibles To Your Map
From CustomCoD Wiki
All the builder has to do for this to work in their level is right-click in the 2D window, choose Destructible, then choose the MP destructible they want to use.
If they don’t use MP destructible assets, the script will assert and tell them to replace the SP asset with an MP asset.
The following is a list of all usable destructibles in MP:
For explodable barrels:
For explodable ammo crates:
The three exploadable barrel types and ammo crates types are essentially all the same.
But you can use either of the three for the same effect.
Please be sure you use ONLY MP destructible assets in your levels.
You can use SP barrels and ammo crates with no problem.
There are currently no dedicated MP explodable barrels and crates.
Source Treyarch Wiki