[darkgael]

There has been and still is continuing confusion about this idea, though most of the necessaries have been addressed here.
You can shoot .410 Shotshells in a gun chambered for .45 Colt....if the chamber is long enough. The Judge and the Contender .45/.410 barrel are examples of this.
It does not work safely the other way around though...shooting a .45 Colt cartridge in a standard .410 bore shotgun. Why? Modern .45 Colt loads use bullets that are .452" in diameter. The bore of a .410 is, well, .410"......and .410 shotguns frequently come with fixed full choke barrels. That reduces the diameter to something around .395".
People will tell you that they do this, that it has been done, that they have never had a problem....OK, "people" do lots of silly things.
Just because you can, doesn't mean that you should.
Pete