I tried this extensively when i was in High school, and didnn't" know betterr" and couldn't afford dollar apiece Breneke slugs. First thing you have to understand is that pressure is CHAMBER PRESSURE. if pressure stayed the same allthe way down the barrel, it WOULD burst the barrel. period. Second, unless you have a verty qweak grip, you can easily mash a shotshell out of shape with a thumb and fore finger. Third, the end away from the crimp is open, so IF THE projectile was squeezed, swaged, mashed, whatever, it would only get longer. If you look at the the pressure curve inside a Rifle barrel the pressue is nearly nothing at the muzzle, much less a shotgun barrel. Normal, ordinary shot shells can almost be "silenced" by using a 40" " Barrel extention" . This again proves the pressure is very, very low at the muzzle of a shotgun. The real danger, as Veral said, would be shooting a second cut shell through a bore that had a partial hull stuck in it.
I remember one of Capstick's stories, something about being bird hunting with a 20 gauge, and having the backbone to wait out a charge, knowing at twenty to thirty feet a charge of #8 would at worst blind a lioness, while a ten feet or less, it would decapitate her. Being as Capstick lived to write the story, the part about the devestation at least is true.
same thing with a shotgun "bang stick" for sharks, gators, etc. No barrel to speak of, but very, very effective on sharks and saurians.Long and the short of it is this stuff works , and works darn well. The very fact that isd "squeezes the in the bore probably makes it as accurate as it is.