Product acquisition policies define legal markets. Policy evaluations require data but prevalence data are not always available. We introduce Legal Firearm Prevalence (LFP), a direct behavioral measure based on the population of firearm licensees in Massachusetts, and argue that it can help evaluate firearm sales and usage restrictions. LFP is not directly measurable in most firearm markets, so we test candidate proxies for LFP in several common research designs, finding that firearm acquisitions are the best proxy in every research design tested. We update the classic study of guns and crime by Cook and Ludwig (2006), finding that choosing an invalid proxy can lead to false research conclusions. We recommend systematic collection and reporting of firearm acquisition data to improve firearm research and inform firearm policy.