In the case of the 9.3mm caliber designation...
the 9.3mm is the actual bullet diameter (.366"). The 8.7 marking indicates the gauged bore diameter (.3425"), which is tight by U.S. standards, indicating that the grooves, if of full bullet-diameter depth would have to be something like .012" deep.
The truth is that German bore and groove standards, as made by various makers in the years before they were standardized across Europe, are all over the map.
Still, there are 2 likely possibilities:
1. The chambering is actually intended to be 9x72mm, which would be determined by measuring the actual groove diameter of your barrel, which should be somewhere around .354"-.355", and the grooves only about .006" deep, which is more likely. That would mean the chamber has either been reamed-out to accept the fatter 9.3mm round, or,
2. The combination of tight bore and deep grooves made it (relatively) safe to shoot the 9.3mm bullet without producing unsafe pressures, though I've never encountered an original German rifle with quite so tight a bore diameter in that caliber.
I suggest that you measure the groove diameter to see what that actually is, and if it proves to be 9.3mm (or thereabouts), simply accept it as another example of the work of those wild and crazy German individualists in the gunmaking trade of bygone days.
PRD1 - mhb - Mike
