Remington Arms ran a test many years ago and filmed it where they dropped a 20 ga shell in a 12 gun then fired it. No barrel rupture. Don't know how many times they ran the test but I saw two different guns where they did it. A 870 and a1100. I got the impression even they were surprised when it did not cause a blown barrel. Perhaps they did it for an in house test in case they got into a lawsuit over a failed barrel or perhaps they were just extreme testing their guns to destruction. Glad they never released that film because you just know some other idiot would try it and not take the safety precautions that they did.
I was standing behind a gentleman on the skeet field, next in line to shoot, when this very thing happened. It was an auto of some kind and blew out just in front of his hand and he got only a scratch from the splintering wood. He had a couple other 20ga carts in the bag so the 20/12 thing seemed like a reasonable explanation. No one bothered to mention how incredibly stupid it was to mix the shells like that.
