IIRC, a properly done rimmed cartridge chamber should also headspace on the shoulder (in a perfect world), but have the rim as a positive extraction structure. I know I have read about overly long necks and throats in 30-30 reamers, especially for cast bullets, so if building/rechambering anything, one needs to research and ask the proper questions before ordering. Remember a sloppy chamber is a sloppy launch platform, no matter how good, or expensive, a barrel is.