The ring boring tool? You place a stack of rings inside and screw the tool together. Using a boring bar and or reamer in the lathe, you can enlarge the center to whatever diameter you need. I use .875 ID on my butt sleeve ring packs but they only come in 0.750 ID. Fiber rings always seem to run...
I have several sets of Carbide sanding mandrels available for $220 each plus shipping. I can do custom sizes as well on request in 3/8-10, Radial, 5/16-14, 5/16-18, and Uniloc.
3/8-10;
.830
.835 x2
.840
Flat 3/8-10;
.840
.845
Ring boring tool sets are $90 each plus shipping and come with one...