From Wikipedia.

*The term "regulated" means "disciplined" or "trained".[160] In Heller, the U.S. Supreme Court stated that "[t]he adjective 'well-regulated' implies nothing more than the imposition of proper discipline and training." In the year prior to the drafting of the Second Amendment, in Federalist No. 29 Alexander Hamilton wrote the following about "organizing", "disciplining", "arming", and "training" of the militia as specified in the enumerated powers:

If a well regulated militia be the most natural defence of a free country, it ought certainly to be under the regulation and at the disposal of that body which is constituted the guardian of the national security ... confiding the regulation of the militia to the direction of the national authority ... [but] reserving to the states ... the authority of training the militia ... A tolerable expertness in military movements is a business that requires time and practice. It is not a day, or even a week, that will suffice for the attainment of it. To oblige the great body of the yeomanry, and of the other classes of the citizens, to be under arms for the purpose of going through military exercises and evolutions, as often as might be necessary to acquire the degree of perfection which would entitle them to the character of a well-regulated militia, would be a real grievance to the people, and a serious public inconvenience and loss ... Little more can reasonably be aimed at, with respect to the People at large, than to have them properly armed and equipped; and in order to see that this be not neglected, it will be necessary to assemble them once or twice in the course of a year.*

If all gun owners need be under the regulation and disposal of a national security body, and are trained by the state, from where do those not so regulated and trained derive their right to own guns?