I recently replaced the VF lens in one of my 2231's (similar to 5231 turret) and found that the longer lens was not capable of focussing at both ends of the zoom range.
By adjusting how far into the holder I inserted the lens (no threads,) I could make it either focus at closest OR furthest zoom (but never both at the same offset).
I wonder if perhaps tweaking the seating of the lens in the holder like i did might allow for focus at full zoom (perhaps the the expense of focus at close/narrow zoom) ?
Of course that means opening it up and potentially voiding warranty, but may be preferable to a trip back to China. Anyone played with the VF lens seating on their 5231 turret or Bullet?...