That's interesting! I have a Mossberg 12 bore shotgun with a barrel less than 20 inches long. I wonder how it would be classified in the UK, or even if it would be permitted.
You would have to have it on your fire arms certificate rather than your shot gun certificate.
I used to have a eight shot pump action Mossberg a few years ago,great fun but it was a 20 inch barrel and open choke ,so it was not much of a hunting weapon