Hello
I want to order a stop ring mold from Accurate Molds, but I need advice on diameters. Nose, bore riding I assume, How much larger than groove diameter should the driving bands be, and the hardest choice, how large should the stop ring be? Does the stop ring do any thing other than prevent the bullet from sliding into the case. If the stop ring acts as a gas seal then the diameter is very important & I will need a chamber cast.
Thanks for any advise
Mike

Mike  everything starts from your individual rifle's chamber and throat/bore dimensions.  As I understand it; in basic theory the bullet base should be a snug fit in the mouth of your fired case as expanded to fit the neck of the chamber. 
The diameter of the "stop ring" itself should be about equal to the OD of your fired case mouth the length of the stop ring band should fill any gap between the forward edge of the case mouth and the rear edge of the leade/throat. depending on how the throat/leade is cut in your rifle you may be able to use a bit of a tapered nose or even a few slightly tapered bands to a bore riding segment of the nose.

I have used the Buffalo Arms standard 8.15x46r stop ring bullet mould but I think that one made especially to fit the way my chamber was cut might potentially be more accurate. So I am planning on getting a custom dimensioned Accurate Mold also, once I get a few fired cases to get an average measurement and I am going to do a cerro-safe chamber cast early next week to see exactly how my chamber is dimensioned.  even with nominally standard cartridge chambering tool dimensions different gunsmiths might cut them deeper or shallower. Original European schuetzen rifles may predate standardized cartridge dimensions as well.  So a chamber/throat cast will guide me to proper case trim length as well as correct diameters for my rifle.

I will be trying a number of different 32-40 breech-seated bullets as well.  I recognize too that these rifles were designed as Offhand rifles rather than benchrest rifles and that a basic bullet/load may offer practical-accuracy enough when we factor in the variables we create as offhand shooters.

further personal thoughts:   I Have absolutely no idea who/what/when/where the Buffalo arms stop-ring was designed for.  However I suspect strongly that it's generic one-size-may/may-not-fit-all has caused a lot of shooters, who are trying to get an old euro-shuetzen working they way they were originally used, to conclude that they don't shoot well.   A look at some of the old german swedged bullet supply house catalogue pages shows clearly that the offered a very wide variety of different stop-ring bullets in differing bore diameters.  It also seems they offered sample packs of differing sizes and styles to find out what shot best in a given rifle.   I'm pretty well convicted that a stop-ring bullet properly designed to fit a specific chamber neck/cartridge-case and throat should be perfectly adequate for offhand competition (within the shooters ability of course )