7.62X54 without the R. I have always been an Outlaw Reloader who reinvents whenever it can be done safely. John Browning made the 45 ACP from the 1895 Mauser 6.5X55 which measures 0.476 to 0.479 at the head of the case, the 45 ACP measures 0.476 inch. The 7.62X54R is 0.479 inch at the head, the same as the 6.5X55. If you decide to form rimless 7.62X54 cases from 6.5X55 brass, you may need to hold the measurement from the face of the case to the junction of the shoulder and neck to possibly 2mm or about 0.08 inch longer and carefully close the bolt on a "crush fit" to make the case headspace properly. Since the extractor does not normally grab the rimless case ( But one of my Moisin Nagants extractor does), then shooting these rimless cartridges is usually a single shot/one at a time proposition. But it is a heck of a lot of fun to do it, "just because I can." On pulling bullets from the steel cases, I usually need to neck size or partial size the case so that my 155 grain spitzers are firmly gripped by the necks of the cases. And sometimes no matter what you do, the necks will split while seating the bullets into those steel case necks. I have never lost a single case though because when a neck splits, that case gets trimmed in a 45 Colt Trim Die to the length of a 454 Casull. And while we are on the 454 Casull/45 Colt, it is also possible to make rimless cases from the 6.5X55 for them too, in a pinch, but each case headspaces wherever its forward edge seats at its deepest point which may be deeper/longer than an ordinary 454 Casull/45 Colt cartridge. Just be carefull and watch what you are doing.
