The semi's headspace is set by the chamber. As 10MM Auto is a longer case than a 40 S&W the front rim will not headspace. The round will go forward and not be hit by the firing pin. Sometimes the chamber is tight enough to "hold" the round enough for the primer to be hit without the case moving. The extractor is some semi automatics also might do this. It would not be something I would want bet my life on however.
Yes, i most certainly would not run any 40 S&W ammo in a 10mm gun for SD purposes, my old P226 in 40 has been very good to me.
