Enact this, and as a former serviceman who swore an oath, I am obligated to stop you at all costs.
Your argument would
a) do nothing to reduce crime or mass killings
b) furthermore, since we have no militia, it is a de facto nullification of a primal right (and no, the National Guard is not a militia, sorry, you don't send a militia abroad to foriegn wars)
c) with the increasing breaches of American civil liberties, we need our guns now, more than ever...
You swore an oath to The Constitution. If it's enacted, it becomes part of The
Constitution to which you swore that oath.
If that oath included a "but only the version in existence right now" clause, then imagine the clash between military and the public when Prohibition was repealed.