Well in my Fictional Setting it's a near Future "Cyberpunk" One. It's not full on People have metal limbs with everything including a mixmaster attached. It's more "Five Minutes from now". Most of the Story takes place in a 100 mile radius of El Paso Texas. Now usually Texas is Quiet compared to Mexico. Normally NFA weapons are rare. In the case of my story you have the Badguys activly hunting the protagonists and are carrying heavier than criminals normally would. Plus one of the badguys has a factory in Mexico churning out little MAC-11 copies. Thankfully most of them are stockpiled and only given out to "trusted associates"
In looking at actual Violent crime figures I'm suprised at how few Violent Criminals are armed.
http://www.bjs.gov/content/pub/pdf/wuvc01.pdf
As far as Twilight 2000 goes I can see the number of armed criminals rising and far more than 1% using Military grade firepower.