|The following is based on Van Buren and has not been confirmed by canon sources.|
2mm EC is a type of ammunition that was to appear in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is also featured in the Van Buren tech demo game files.
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While it doesn't appear in the Van Buren tech demo, it is still present in the game files and can be spawned with the console command CreateEntity Ammo2mm_EC.amo.