The 2nd Amendment begins with an explanatory statement, and might as well have begun with the word “BECAUSE”. “A well-regulated (trained) militia (a military force that engages in rebel activities in opposition to a regular army) being necessary to the security of a free (as opposed to a totalitarian) state, “THEREFORE” The right of the people (me and you and especially the weaker among us) to keep and bear (military) arms (typically carried by infantry soldiers on the battlefield) SHALL NOT BE INFRINGED.  Typically, the militia also serves as a very informal way for the local sheriff to organize, train, and get to know the armed individuals in his county while keeping a very informal, friendly eye on them.
 
So, our forefathers (and their equally freedom-loving wives) knew precisely what it meant when they read the 2nd Amendment…. So therefore, I absolutely oppose any further restrictions on the sale of military grade weapons. To begin with, it is impossible to restrict supply in a free society. For example, if our enemies wanted to import a briefcase nuclear bomb, all they would have to do was to hide it in a bale of marijuana, right?  It is easy enough to buy weapons at a local gun dealer, and of course they do require ID and a background check, and they do not sell to children. Nevertheless, anyone with basic machining skills can make firearms all day in the privacy of their own homes, or businesses, and would do so if it were not for the ready availability of such arms on the black market.
 
From WWII until today, 18-year-olds are eligible to join or be conscripted into the military.  I learned to shoot at age 16 in the basement of the local YMCA, and it helped me when I enlisted. A firearm is just another tool like a chainsaw or a car. All three can be used to kill.  Knives are very handy and have the additional benefit of being silent and easily concealed.  A sharp pencil will do if necessary, and I am sure you would not be very surprised at the number of household products that can be made to go boom!  How much restriction do you want?   Some villages overseas were reduced to a single knife chained to a stump in the center of the village for cooking purposes.
 
People have a responsibility to secure their weapons and keep them out of the hands of children. Schools have a responsibility to regulate entrance to their buildings, check students as they enter, and know how their students are feeling and doing at all times. Public venues have similar responsibilities and may allow or prohibit entry to those who are armed.  This has always been the case, but sadly it is not always done.  People who do not take their responsibilities seriously should be penalized, but not the rest of us. Our 2nd Amendment rights undergird and protect our 1st Amendment rights and all the others enumerated and not enumerated in the Constitution.