I'm almost positive that the 300 Blackout won't chamber in a .223 or 5.56 chamber because the 30 caliber bullet is too big to fit in the end of the chamber where the 22 caliber bullet would go. I think the 300 would get about 2/3rds of the way into the 223 chamber and stop.
I've shot a lot of 38 special through a 357, it's a lot cheaper...
As I understand it, it depends on how the chamber is cut. People say it happens, and the video seems to demonstrate it, but I definitely understand your argument.