The link on the Birmingham Proof House website to this research report doesn't work but I found it on a British website "Pigeonshooter" absent the artwork. The wildly varying pressures in these tests tells me that sometimes the gun would be unhurt, sometimes bulge a barrel and sometimes suffer catastrophic failure. The odds look poor enough that I'll do all I can to prevent a 20 gauge shell getting into a 12 gauge gun or a 28 gauge shell getting into a 20 gauge gun.

http://pigeonshooterhomepage.webs.com/safteyandthelaw.htm