The video that will not die. I belong to many firearms and survivalist forums, and this crap has been circulating for maybe three years now.
This idea started years ago with paper hull shells, and you may get away with it using those. But plastic shells are a different deal.
Try this - try to fit the nose end of a loaded shell in the muzzle of your shotgun. Didn't work, did it?
Now imagine that same diameter, 1 ounce (or more) mass trying to fit through there at 1200 feet per second. You are darn right there will be a presure spike. A big one. Normal chamber pressure for most gauges is around 14,000 psi. That pressure, in a milisecond, will vastly increase.
You are holding this next to your head.
You might get away with this once, twice, five times, but sooner or later you will bulge the barrel. Or split your barrel. Or worse.
This ranks right up there on the stupidity scale with firing a 45 Colt round through a 410 shotgun.
To give an idea of how tricky interior ballistics can be, the chamber pressure of the .223 Remington runs from 40,000 to 50,000 psi depending on powder and bullet weight. Simply firing 5.56 NATO in a .223 chamber will increase this by 10,000 psi. This is why manufacturers of rifles with .223 chambers warn against firing 5.56 NATO in them.
Bottom line - firearms are designed around the ammunition to be fired. Fireams designers and engineers know what they are doing, and I'm going to stick with what they recommend. You can do as you wish.