and you're correct about the chamber- just beyond it is what's known as a "forcing cone", that moves from chamber diameter to bore diameter... the plastic hull will deform, but will resist & build more pressure... again at the choke, if present...
in the video, he said his shotgun had a modified choke...
and on a pump gun, that's why I was curious about a controlled-depth cut, rather than the slit that he made hanging by 1/8" or so... may hold together in a pump... what's the worst that could happen?
neat trick, anyway...