Probably nobody can answer this question here because most users are from the US but I wonder what qualifies a shotgun to be non-restricted or prohibited.
Example: Mossberg 500 could be obtained with a shorter barrel and/or higher capacity magazines etc. Why would a magazines capacity be limited to 3 shells for hunting instead of 5 for example (if you take the plug out)?
In general, what qualifies a shotgun to be illegal? And why?
In the U.S. there is some federal regulations involved when hunting water fowl. They require a plug put in the magazine to limit the total capacity of the gun to 3 shells. There are other regulations as well and variances of them depending on what state you are hunting in.