The right to bear arms was clearly intended for the able bodied and mentally competent. The Amendment addressed the issue of both national defense and the guaranteee that the government would not attempt to erode Constitutional liberties. In any just government, the adminstration of these guarantees reasonably allows for some interpretation and common sense.
For instance, the right to "bear arms" might reasonably be construed to extent to automatic weapons, but not to atomic weapons. It may be reasonable to own a cannon, but does the right to bear arms mean that you can have your own fleet of battleships? or guided missile destroyers?