The problem is not simply black market weaponry. It is the fact that many commonly purchased firearms may be easily modified into fully automatic weapons.
I, personally, am against banning firearms in any way. People kill people. That's just human nature. Take, for example, Japan. They have a complete ban on firearms. Does this stop murder or reduce the crime rate? Not by a long shot. It just means there are a hell of a lot more stabbings and beatings.
Would it change anything if blackmarket firearms could no longer reach our shores? No.
You should also keep this in mind. More devastating weapons are shipped out of this country every year than come in.