The principal reason Georg shortened the cartridge was to make room for the recoil spring in the grip. The Borchardt recoil spring resides in the rear extension that also contains the roller for unlocking the action.
There are no significant differences in diminsions between the Borchardt and Mauser cartridges. The Mauser version was a hotter load, too spiffy for the Borchardt. Actually identifying a Borchardt cartridge today is supposed to involve the details of how the neck is crimped onto the bullet.
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