[caz223]

As for the Glock 20 being the only 'magnum' auto, I have two words for you.
Desert.
Eagle.
I have many 10mm handguns, and the glock isn't the only game in town.
Anyway, a 10mm round won't chamber in a .40 caliber weapon, not enough room in magazine/chamber.
If you put a .40 in a 10mm autoloader, it may fire, it may even extract, but it certainly isn't advisable except SHTF situations.
It prolly won't have a very good gas seal, may not even have enough oomph to cycle the slide.
Now, if you happen to have the smith 610 revolver, and a handfull of moonclips, you can shoot .40 ammo in your 10mm weapon all day long with no problems.
That is one exception.