In most jurisdictions, there will be a single part that is legally the gun. That will be the main part that any licensing and registration will be attached to.

In most jurisdictions, there will be a single part that is legally the gun. That will be the main part that any licensing and registration will be attached to. In America it’s the lower receiver, but in some countries it’s the barrel. Once you’ve got the registered part, everything else can be purchased as parts. For the most part if it’s illegal to own, it’ll be illegal to buy as a part (e.g. unregistered automatic sears in America, or high capacity magazines in some states). The one exception to this will be jurisdictions with restrictions on “assault weapons”, such as California. In these jurisdictions weapons become illegal based on their configuration; adding a pistol grip to a weapon with a removable magazine might make it illegal, for example. As always, do your own research and ask a lawyer.