This is why the purpose of the Second Amendment is not to guarantee a personal right, but to guarantee a ready defense for the common good: Because if each man has a state-of-the-art weapon already, it is easier to assemble an army on short notice and get them started on training. The “well regulated Militia” part of the amendment is not superfluous. In this letter, Washington is complaining about short enlistments, but the point is that a militia in itself (as conceived, for example, by the anti-Clinton insurgency during the Bumbling Camo-painted Doofus in the Woods Campaign of the 1990s) is not reliable for military purposes:
To place any dependence upon Militia, is, assuredly, resting upon a broken staff. Men just dragged from the tender Scenes of domestick life; unaccustomed to the din of Arms; totally unacquainted with every kind of military skill, which being followed by a want of confidence in themselves, when opposed to Troops regularly train’d, disciplined, and appointed, superior in knowledge, and superior in Arms, makes them timid, and ready to fly from their own shadows. Besides, the sudden change in their manner of living, (particularly in the lodging) brings on sickness in many; impatience in all, and such an unconquerable desire of returning to their respective homes that it not only produces shameful, and scandalous Desertions among themselves, but infuses the like spirit in others. Again, Men accustomed to unbounded freedom, and no controul, cannot brook the Restraint which is indispensably necessary to the good order and Government of an Army; without which, licentiousness, and every kind of disorder triumphantly reign.
George Washington, 24 September 1776, to John Hancock; quoted in The Genius of George Washington (1977), by Edmund Morgan (p. 47-48)
This could just as well be an indictment of the US policy of sending National Guard troops overseas to fight for other countries, except that the National Guard do not end up with short enlistments in those cases. Washington was not against citizen soldiers; he just wanted enlistments to last more than a few months, and he wanted officers who were “gentlemen” rather than petty gang leaders.
A counterpoint is that a militia is perfectly suited for certain purposes, such as a guerrilla defense against invasion or domestic terrorism during a civil war. Sebastian Junger writes this in War (p. 83):
The fact that networks of highly mobile amateurs can confound — even defeat — a professional army is the only thing that has prevented empires from completely determining the course of history.