It's my understanding the "R" desiginates "rimmed", which means the cartridge case has a rim of a diameter larger than the case body which protrudes from the case like the rim on a shotshell. This allows for extraction in a double barreled rifle. The 7X57 case has a rim the same size as the case body with a rebated (smaller diameter) groove between the rim and the body, into which the claw (or other) type, of a rifle with a bolt engages for extraction. Example: the 45-70 is a rimmed cartridge, the 30-06 is not. Hope this helps. Bill
