16 mm / .625 inch blowgun cones. The hole is slightly conical, 3 mm (.118 inch) to 2.4 mm (.094 inch), for a tight fit.
Before you use the cone, first remove the excess plastic powder from the hole with a sharp object (example: the sharp end of a dart). The last bit of powder can be blown out through the hole in the back of the cone. The hole seals when you put a dart in place.
The cones weigh about 0.35 gram or 0.011 ounce.