Original Smooth Twist is 1 in 16 as Bob said, just be aware that the actual resulting pellet twist rate is much, much less due to the way it is launched in a smooth section of the barrel and then the pellet strips through the rifled section and picks up a small amount of spin. It has been measured at something like 1 turn in 13 _feet_.
That is an interesting thing about the diabolo shape. It’s flare stabilized and does not need much spin to stabilize it. In fact this slow spin tends to be beneficial in some cases at longer range because spin rate decays much slower than the forward velocity. So for example, what happens at 100 yards is the pellet is moving much slower than when it left the muzzle but it is still carrying most of its spin, and that condition can cause the pellet to destabilize. It’s one of the common reasons for spiraling at a distance.
