The Ultracite .38 Round is an ammunition type in Fallout 76. No weapon fires it out of the box, but mods convert 2 weapons to it.
It can be crafted from Gunpowder, Lead, Stable Yellowcake Flux, Steel and Ultracite; one craft makes 72.
A True Believer: +20% Damage with Licensed Plumber, +20% Damage once Craft or Scrap Pipe Weapons is complete