Th cone geometry in the API returns a semielipsoidal shape instead of a cone:

Cone(float64, float64) Shape
3D Cone with diameter and height in meter. The top of the cone points in the +z direction. x

The equation in the file shape.go is not the one of a cone: https://github.com/mumax/3/blob/e45ea8b62bc544ec6df67648c1b72a3bbfd4f17f/engine/shape.go

// 3D Cone with the vertex down.

func Cone(diam, height float64) Shape {

return func(x, y, z float64) bool {

	return z >= 0 && sqr64(x/diam)+sqr64(y/diam)+0.25*sqr64(z/height) <= 0.25

}

}