A sealed criminal record in Massachusetts does not mean the record is sealed for all purposes. While a potential employer would not be able to view a sealed record, there are exceptions. One example of an exception is for a "legitimate law enforcement purpose."
Even if your criminal record is sealed, an application for a firearms license in Massachusetts is deemed to be a "legitimate law enforcement purpose", and the reviewing agency would be able to see that there is a sealed record.
In addition, if the sealed record were a conviction for a disqualifying crime, you would remain ineligible by statute to obtain a license. In addition, if the conviction is not an automatic disqualifier, the existence of that sealed record (and police records pertaining thereto) may still be used by the issuing department to address the issue of the applicant's suitability for a LTC.