Aparently the guns were made with some leftover parts as mentioned above with the balance made to fit the scaled up barrel. The wall thickness at the muzzle is around .160.
When Belgium still had colonies in Africa, the story is that they sent these there for the natives to use. I have seen that story a number of times including in the Stoeger catalog (the same people who said this was a 4 bore)...

Any new and possibly more factual info would be appreciated.

Still want to shoot it and feeling more confident as it is only a 6.