Stop with the lines that the anti-gunners use. Bad guys can get them right now, the NFA '68 can't/won't stop them from getting them. I used to know a bunch of dealers (drug not gun) that I could get full autos from for $100. I have since moved to another state and do not keep in contact with them.
The reason that most gangbangers don't carry them is that they are hard to conceal (even the ones with folding/collapsible stocks and short barrels) even in the fashion of de dium (baggy clothes) and if they manage to conceal them the guns are hard to pull out in a pinch. That is why the true gun of choice is the handgun... easy to conceal and to pull out.
That makes some sense. But I still point to Nuevo Laredo where automatic weapons are being used extensively by the gangs there. So there's another factor in play.