The Massachusetts Firearms Records Bureau recently published administrative data covering the universe of legal firearm transactions in the state. We use these data to validate state-level background checks as a proxy for firearm transactions and show that historical trends in transactions within Massachusetts align with the rest of the United States. Using auxiliary data from a national survey, we show that the Massachusetts dataset can detect patterns in the demographics of both gun ownership and type of firearm purchased. Our analysis suggests that this dataset is a promising source of information for studying interactions in the market for legal firearms.