I don't know that you need one. I bought this configuration to use at 3 gun matches, as it will hold 10 rounds total. With the object being to reduce the time spent loading and to get back onto the next target as soon as possible.
It came with an open chock and I wanted to tighten up the pattern so that I could shoot at longer distance with buckshot. I can still keep all 9 rounds of 00 on the target at 30 yrds, but I was trying to keep them all there at 40 or 50 yrds.
Clear as mud? That why I was asking.