That being said, it would have done nothing to stop Lee Harvey Oswald, Charles Whitman, Sirhan-Sirhan or James Earl Ray.
Should one also have to prove he is a "responsible law abiding adult citizen" before being allowed to speak, worship, assemble, remain silent, be free from unreasonable searches, or from cruel and unusual punishements?
That being said, it would have done nothing to stop Lee Harvey Oswald, Charles Whitman, Sirhan-Sirhan or James Earl Ray.
If you admit such laws won't stop these sorts of things, then what is it you are attempting to accomplish by supporting them anyway?
If the government dealt with serious law breakers to start with, they wouldn't be buying guns since they'd all be hanging from ropes or busting rocks.
This is a classic case of the Hegelian Dialectic at work. Problem, reaction, solution. The gov't creates (through action or inaction) a problem, manipulates the public reaction, and finally proposes a solution which involves more government and higher taxes. Not only this, but their "solution" leads to another "problem", starting the cycle again.
The Constitutional issue is known as "prior restraint" alternatively understood as "guilty until proven innocent" and is specifically prohibited by the Constitution.
History is littered with examples of government arbitrarily and capricously assuming guilt and acting on that presumed guilt with extreme prejudice.
One oft referenced example is, of course, the policies of the duly elected German Government toward various segments of their population such as Jews, Gypsies and other non Aryan people.
The possession of armament by the general population (including military weapons such as shoulder fired missiles and various explosives-grenades,land mines etc.) is intended as a check and a balance on the capability of a Government to wage war on its own population.
Hope this helps.
Best regards,