There is no way to over stress the extractor shooting 40 S&W in a 10mm
The theory I've heard proposed is that when you shoot .40 S&W in a 10mm barrel, you are relying 100% on the extractor hook to headspace, and when the firing pin strikes the primer, that's stress on the hook that isn't present when a round is headspacing in the forwardmost part of the chamber on the mouth of the cartridge.
