Interesting pic Rockdoc. This very graphically shows the difference between a .410 & a true 36 Gauge. I quickly measured a fired RP .410 hull & got .480" for the head just in front of the rim & a rim diameter of .533". Thus if dropped into the bore of that 36 open end first it would be stopped at the rim but the entire hull would simply rattle around in the bore.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra