What kind of gun is it? There are smooth rifles out there that left the factory that way.
Many .22 birdshot guns were produced back before the 68 gun control act and like
ADP3 said it would have had to be registered if the barrel is shorter than 18". If not registered and it were mine, I would either re barrel, sleeve it ,part it out or scrap it.
It would take an act of congress and many $$$ to make it legal today if even possible.
Some bores have a POLLYGONAL (sp?) type rifling that looks almost like a smooth bore. I`d check that possibility.