Fusion magazine (Fallout 76)

The fusion mag is a weapon mod for the alien blaster

Crafting
Weapon modifications will modify an existing weapon, and any modifications previously equipped on the weapon will be destroyed, not unequipped. Loose mods cannot be crafted. For modifications unlocked through scrapping, the corresponding weapon must be scrapped in order to learn how to craft it.

Location
Craftable at any weapons workbench.