There is currently an issue with the game's loading order which prevents some masterbundle overrides, hopefully this will be fixed (thanks to WWTC for finding it)

"Master Bundle Overriding" is a method that allows one asset to use another asset's unity prefabs.

For example, you can create a custom version of the Maplestrike with different statistics, without having to rebundle the whole thing in Unity. This also works with workshop mods.
It is also useful when optimizing masterbundles, when several assets use the same prefabs (this is often the case with quest objects).

• Masterbundle overrides will only work when directed towards a Masterbundle; the Unity3d files won't work.
  
• While this method is good for servers who want to tweak the statistics of a mod, the base Masterbundle is still needed for this to work. So you would want to have both the original mod, and the mod containing your custom assets.
  
• Considering the point above, masterbundle overrides are the best ways of adapting mods to a server (since it does not involve reuploading the assets themselves).

The easiest way to do so is by copying the dat files of the asset you want to "use", and paste it in your mod files.

If you do not have such files yet, you can create a folder named "Bundles" in any of the maps folders (Unturned/Maps), except Tutorial for some reason...
Having a Bundles folder inside a Map allows for easy workshop uploads, no masterbundle conflicts, and not loosing your custom assets among the many vanilla assets.
In my example, I created a Bundles folder inside of Russia :


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Paste the dat files inside a folder created within the Bundles folder created earlier (it does not have to be at the root of the Bundles, as long as it is part of that folder).
In my example, the folder is located in Russia/Bundles/Items/Weapons/ :


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Rename both the main dat file and the folder containing it with the same custom name.
In my example, both the folder and file are named "BundleOverrideDemo" :

This is the easiest as it is usually the name/description of the asset.

BundleOverrideDemo.dat in my example

In addition to the statistics you want to tweak, there are things you must change in order for your asset to work :

- The ID has to be unique;

- The GUID has to be unique (removing the whole GUID line will make the game generate a new one);

- Master Bundle override lines :
There are 2 lines :

Master_Bundle_Override -> Specifies which MasterBundle contains the asset's prefabs you are using;

Bundle_Override_Path -> Files path from the MasterBundle.dat file to the asset folder

In my example, I would like to use the Swissgewehr prefabs, which are part of the core.masterbundle :

Master_Bundle_Override core.masterbundle

The path to the Swissgewehr files from the core.masterbundle file is : Items/Guns/Swissgewehr :

Bundle_Override_Path Items\Guns\Swissgewehr

So here is the final result :


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If your masterbundle override is located inside a mod that already uses a masterbundle, make sure you add the Exclude_From_Master_Bundle line as well.

Now your asset should work;