“Extremely rare 15mm EC ammunition pack more of a punch than 2mm EC one, which is very bad news for armor.”
15mm 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.
In the game, it was expected that this would be a very rare ammunition used only by the 15mm ARTEMIS rail gun.
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 Ammo15mm_EC.amo.