The right is general. It may be supposed from the phraseology of this provision that the right to keep and bear arms was only guaranteed to the militia; but this would be an interpretation not warranted by the intent.
The militia, as has been explained elsewhere, consists of those persons who, under the law, are liable to the performance of military duty, and are officered and enrolled for service when called upon … [I]f the right were limited to those enrolled, the purpose of the guarantee might be defeated altogether by the action or the neglect to act of the government it was meant to hold in check.
The meaning of the provision undoubtedly is, that the people, from whom the militia must be taken, shall have the right to keep and bear arms, and they need no permission or regulation of law for that purpose.
— Thomas Cooley
(1824-1898) 25th Justice and a Chief Justice of the Michigan Supreme Court (1864-1885)