Being caught with such a firearm in the Glorious Commonwealth of Massachusetts will land you in jail for 1 year. Mandatory.
Gad, I hate this frakkin' place.
What if you purchased a gun prior to becoming a Mass resident? Does Assachusetts require the registration of all guns owned by its residents?
I used to live in (The Peoples Republic of) New Jersey, which is a lousy place to be a gun owner, and even it didn't/doesn't have such a requirement - of course, you can't own an "assault weapon" but that's a different subject. I could go back to NJ with a bunch of handguns that I bought while a non-resident, and not need to register any of them (but if I wanted a new one, I'd need the approval of the local PD). There's little liberty there, but it isn't a total tyranny (though that's being worked on, diligently).