I'll unconfuse you.
There are certain situations that require "reporting" of transfers - pretty much anything except moving into MA from out of state.
Records are kept (unreliable for old transfers).
The onus on reporting a transfer is on the seller. It is not an offense for a lawful recipient to be on the accepting end of a transaction that the transferrer failed to report - the only offense committed is by the transferring party.
And, to top it off, there is no such offense in MA as "possession of an unregistered gun".