Jim,
One of my 21s has a left barrel that was opened up, and is correctly factory overstamped, from WS2 to IC. There is no parallel section, indicating that they did not simply hone at the muzzle. I don't know if they retained the original angle (shortened the choke section), or opened the choke by reducing the taper. In any case I agree that the usual approach is to leave a parallel section.
My VH 12 was opened (by Mike Orlen) from .034 to about .014. There is a distinct parallel section. It now throws very nice LM patterns despite the parallel. So I wonder how much parallel section is needed in order to reduce choke effect?
Sam
