They sell them. Check them out in the Cabela's catalog or website. They are double the cost of the standard chokes and I just have no experience with them as to their effectiveness. That's why I posted.
I hunt with an NEF 12 gauge bull barrel fully rifled and with Remington Copper Solids it is VERY accurate having taken deer at 135 yards, and hit right where my range practice told me it should hit.