These cones are for a .625in "Big Bore" blowgun, so they have an outer diameter of 15mm (.591in). The attachment hole tapers down from 3mm to 2.5mm, so that it will fit snugly on a standard bamboo skewer.
Spiral fins are featured on the front to act as fletchings, spinning the dart as it flies. The fins on the back utilize the blowing force to add additional spin to the dart. A dart with spin will have increased aerodynamic stability compared to one without.