Load 10% back from the 8X57 IS loads for the JR. You can load with standard 8X57 dies, as used for the .323 diameter bullets...usually. I substituted the .323 neck expander with one from a set of 303 British dies I have, so the necks don't get expanded after sizing. It just keeps things lined up a little and holds the de-priming pin in place. You can also take an expander for a 32 Special and put the rod and expander in a drill. Use a medium fine emery cloth and burnish it down to .317.
We did this for many years, before we could buy standardized 8X57 JR dies with the smaller expander for the .318 bullets. We also shot a lot of old Remington 170gr 32 Special bullets in the J bore guns; they are .321 diameter. Most J bore guns have a .319 to .321 bore, but not all. Since the bullet sizes into the bore early on, before pressures spike, this works well.
I actually got better accuracy in my J bore guns with the 32 Special bullets. I've done this in 25 to 30 or so drillings and other combo guns since 1959.
Use Reloader 15 powder, also. It seems to have been made just for the 8X57.
Hawk and Woodleigh make good .318 bullets. I would and do use theirs nowadays.