Eugene Volokh: Naturalized citizens and guns: A victory in Massachusetts

From Ngo v. Evans (D. Mass. Nov. 7, 2014) (emphasis added):

To be eligible for a license to carry firearms in Massachusetts, an applicant must provide the appropriate licensing authority with, among other things, proof of residency, age, and U.S. citizenship. The constitutionality of these requirements is not at issue. [The citizenship requirement is likely unconstitutional, at least as to noncitizens who are permanent residents, see, e.g., this post and this one, but that is a story for another day. -EV]

 More Here