[mrerick]

The 9mm Luger round headspaces on the mouth of the cartridge. There is a ridge in the chamber that the mouth of the cartridge rests upon.

The toggle train length determines how close the breech face is to the head of the cartridge when it rests in battery. As it grips the cartridge in the extractor, it must be long enough to press the cartridge mouth against that ridge in the chamber.

As long as you're not trying to mix components from different frame length guns it will probably be correct and within tolerance. You can do measurements if needed, or get a set of 9mm headspace gauges (especially the "Field" gauge) to ensure it's right. The train should not close on a field gauge.

Marc