Re: only private owners
I thought a shotgun certificate only covered smaller bores - I recall 2 inches being a limit, but even that may only be for muzzle loaders.
Above that you probably need a proper firearms certificate; check with your local FEO prior to purchase, just to be sure.
For example, even if you're only firing blanks, anything above 1 inch requires a full firearms licence. I can't see them allowing a non-deactivated 120mm breech loader on the streets on a shotgun licence.