Nanofactories are the reason why Forge Worlds and New Planets can pump out Billions of Tanks, Valkyries, Trillions, of Armour and Guns, Millions of Titans and Space Marines. And work on the principle that any material, even air, can be converted into Metal and many other materials, the Nanomachines convert the material into metal and then it is used in the Forge World or New Planet.
There are also miniature Nanofactories for guns that make them have virtually infinite ammunition. However, in the case that miniature Nanofactories are not researched yet, Quadrillions of Bullets are made on a Forge World or New Planet.
Note: In Real Life, Nanofactories and 3D Printers may be the new way Warhammer Miniatures are created. In 2025, 3D Printing will be worth 12 Billion and 8 Billion in 2020. As 3D Printing is a rapidly growing industry. If Wargaming is still around in 2040, Nanofactories will be behind Millions of new Warhammer Figures.