Multi-state residency is tricky, since dealers in states other than MA tend to rely on a driving license or similar to prove in state residence, and do not accept that state's carry permit. If you have a NH driving license, legitimate part of year residency, and a MA LTC with your MA address you coule easily buy in both since MA dealers tend to accept the MA LTC as full and complete documentation of legal purchase status (verified via MIRCS) and residence.
You will likely find that attempting to either argue the law or prove your part time residency status by showing a deed or closing statement to many dealers to be an exercise in futility. They view themselves as authority figures/professionals, will and certainly are not going to risk their FFL on some less intelligent punk amateurs (that's what they see you as) interpretation of the law.