First of all closed bolt guns are much more difficult to convert to full-auto than internet rumors would lead you to believe. Kitchen table conversions are really likely to blow up in your face and guaranteed to run away or jam up. Gangbangers are not running around with fully automatic weapons. They are running around with cheap weapons.
Crimes committed with illegal machine guns are extremely rare. Crimes committed with legal machine guns are even rarer.
Machine guns are expensive to buy and even more expensive to run. If you can't afford the $3200 for an M11/9, you certainly can't afford to shoot it much.