Usually on the box they come in, it will tell you the diameter. If you no longer have the box, take an old sabot that you are sure of the size. Green sabots are .429-.430 black sabots are .451-.452 and the orange are .457-.458. Take the bullet in question and fit them into some of the old sabots. They should fit firm. Also if you put too smalla projectile like a .430in a larger sabot like a .452, it will really slide/fit realeasy down the barrel.