I think Bryantanthonydp is referring to the law relating to cartridges rather than shotguns. Firearms Law - Guidance to the Police says cartridges containing 5 or more shot, none of which exceeds .36 inches are exempt from certification (S.1 otherwise). As said above a shotgun shooting round ball is still a shotgun.
On the practical side, if someone posts legal advice then it would save time if they also gave the Act/Section or source wherefrom it is obtained. That way it could be quickly verified without argument.
Steve.