The plain meaning of the second amendment, in this context, is that _anyone_ is allowed to own arms, because everyone is expected to take responsibility for defending themselves and the community. There's also another layer; "militia" is as opposed to "standing army", and "a well-regulated militia being necessary to the security of a free state" thus means that the militia (that is, the population at large) needs to be armed well enough to resist tyrannical impositions by a standing army.