They are typically series of miter-cut, off-centered reducers... never concentric, but sometimes eccentric. The reducers themselves are irregular conic sections whose miter cuts are supposed to be circular. Regular cones do not produce circular cuts when cut at these angles.
To that end, it would be ideal to get an accurate model of the bend in question and measure the diameters at the miter cuts/welds. You could then model a miter bend of that diameter at each weld point.
It's worth mentioning that miter cuts needn't necessarily be at regular intervals. E.G. I've seen one comprised of 3 pieces (where Dave shows 4), but the miters were supposedly at 25° and 72°, instead of 30° and 60° as one might expect.