"They" are correct about making the cases from 9.3x74R. I do.

If your bore is a little less than .366, try the .358's with a 1/8" cork gasket material wad under the bullet (use a 3/8" wad punch so it's a tight fit in the neck).

I have a 400/360 with a .372 groove diameter, and it shoots the PRVI 285 .366's wonderfully with the cork wad.