At the risk of being all wet, I'd like to make a couple of comments about Luger cartridges.
The 7.65mm or .30 Luger cartridge is, of course, a bottle neck design. The smaller front portion of the bullet/neck part makes feeding similar to throwning a marble into a tin can. The chamber mouth is huge in comparison to the front part of the cartridge and, once the small front has entered the chamber, it guides the larger body into the cavity. Providing that the cartridge is powerful enough to work the action, jams with the .30 Luger cartridge are rare.
Now comes a demand from the German military that a larger diameter bullet is required because the 93 grain bullet of the 7.65mm is inadequate in stopping power. (This was also a criticism leveled by the US Troop Trails reports.) Suddenly, the diameter of the bullet almost fills the chamber mouth and reliable feeding becomes a problem that wasn't a concern with the bottle neck cartridge.
It's always been my personal theory that the taper of the 9mm case was an intentional design feature to facilitate more reliable feeding and functioning of the pistol. The test bottle neck 9mm cartridges may have been an attempt to permit a straight side case with the feeding reliability of the .30 caliber cartridge.
On another point, in my experience the problem with cartridges in the Luger mag have generally centered around the bullet snagging on the mag catch hole. The design of the bullet nose; the bullet nose material and OAL of the loaded cartridge all seem to play a role in how well the cartridge feeds up through the magazine.
Finally, Hugh may well be correct in thinking that my problems with the C-96 that I had rebored to 9mm resulted from a badly worn chamber. The pistol was one of the shot-out Chinese imports and it certainly had seen better days.
These are just a few random thoughts that I thought I'd share.
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