I would be quick to differentiate weapons used by our military from normal goods. Even if the weapons could be manufactured more cheaply elsewhere, production needs to remain domestic in peacetime so that we can ramp up production accordingly in the event of war.
That's not to say that private companies couldn't be used as supplemental production during wartime(like Winchester, Harrington&Richardson, and International Harvester were for M1 rifles), but our primary means of R&D, testing, and production need to be controlled directly by the military.
Wer mit Ungeheuern kämpft, mag zusehn, dass er nicht dabei zum Ungeheuer wird. Und wenn du lange in einen Abgrund blickst, blickt der Abgrund auch in dich hinein. -Friedrich Nietzsche, Jenseits von Gut und Böse