.26 in (6.6 mm)
6.5mm Bergmann Wikipedia
The 6.5mm Bergmann is an unusual centerfire cartridge produced for very early self-loading pocket pistols. The case is bottle-necked and steeply conical, and headspaces on the conical case walls. Early versions were made without any rim or extraction groove; and relied upon blow-back for expulsion of the fired case from the chamber. Later Bergmann pistols provided an extractor requiring a groove which produced a semi-rimmed case.