Ebjonnes,
If it has a rifled barrel it is a rifle or pistol (depending on bbl length, o/all length and stock configuration) and if it has a smooth bore it is a shotgun. (unless it fits into the catagories of reproduction, historic, or antique firearms. You may also need a license to build or modify a weapon to this configuration. Be sure to check out your local, state and federal laws.
PaulS