i live in mass and you have to register firearms...
Well, sort of, but not exactly. Before I got smart and moved over the border to the north, I spent most of my adult life in the People's Republic. In Mass, you have to file an FA-10 form with the state when you purchase a firearm, either from a dealer or a private seller - typically it's the seller who submits the form - documenting information on both parties as well as on the firearm. In typical Massachusetts fashion, the form is filed with the Criminal History
Systems Board - gotta love that. Although the state claims otherwise (and of course they'd never lie), that amounts to de facto
registration for new purchases.
There's no requirement, however, to register guns that were purchased prior to the start of the FA-10 submittal requirement, so it's perfectly legal to own unregistered guns, and any old fart gun owners (like me) usually have a number of firearms that aren't "registered" with the state.