The face of the breechblock is indeed recessed, but this in no way prevnts the cartridge from sliding up the breechface. The cartridge starts to slide up the breechface at almost the same time it begins to enter the chamber. As the cartridge continues up the breechface, it begins to push the extractor up as the back of the cartridge case and the extractor are on the same plane ahead of the breechface recess. As the cartridge moves higher on the breechface it drops into the recess and the extractor snaps down over the extractor groove in the cartridge. The extractor does not jump over the cartridge rim, but actually snaps into the extractor groove as the cartridge seats into the breechblock recess. The cartridge case begins to push the extractor up before the cartridge case is half in the chamber, and the extractor is fully raised before the cartridge is finally seated. This can be fully examined by slowly cycling a dummy round into the Luger chamber.
At the speed of the toggle assembly during firing, there is no way the cartridge itself is going to jump ahead of the breechblock into the chamber and wait on the breechblock and extractor to seat over it.
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