I believe the chamber of the .410 is over .46". It is not difficult to understand how a .454 Casull would work in it. There would be a lot of extra room for gas expansion before the bullet were to reach the forcing cone. I bet if they could jam a .460 S&W Magnum into the chamber of that gun and fire it there would be a different end to the story.

Instead, what I find fascinating is that the choked muzzle isn't bulging, cracking, or even stretching. Fosbery said he tried bullets of varying hardness in his rifled choke (aka Paradox) and they all seemed to work quite well despite the fact that some of those bullets were very, very hard. Some of the Paradox gun skeptics of his day could not believe that a large bullet could pass through the choke without damaging the muzzle. Fosbery claimed no deformation, even in barrels that had fired thousands of rounds. It would be interesting to watch a slow motion video of a .45 caliber bullet passing through the muzzle of the .410. We would see what is actually going on.