It seems silly to say it in light of history, but Georg goofed. When the German military told him that 7.65mm was too small, they mentioned 9mm. But at that point, he knew only of cartridges that were supported on the rim or on the shoulder, so he first tried necking up the 7.65mm cartridge but leaving a shoulder for case support. That didn't work as there was simply not enough shoulder. Then he hit on the idea of supporting the case on the case mouth, one of the great ideas in handgun design.
But instead of going to a bigger caliber and using a straight case, Luger decided to keep 9mm*, which meant that the case had to be tapered, and that was a mistake. The Luger pistol worked OK with the tapered case, but other guns, especially SMGs with long magazines, have had problems ever since.
After John Browning, who had had to go to a semi-rimmed case for support while getting the rounds to feed through a magazine, saw or heard of Luger's idea of support on the case mouth, he went to that system (in the .380 and .45 ACP) and never looked back. But he was smart enough to make those rounds straight case, reducing feed problems.
*Probably because that was what the military mentioned, and in the Germany of Georg Luger's day, what the military wanted, the military got. Plus of course, the normal desire to satisfy a customer in position to award a huge contract.
Jim
