You can obtain them, from the very limited supply on the market, but they are very expensive and the process is complicated. Legal full-auto firearms are basically non-factors in gun crime.

Semi-automatic lookalikes (like the bazillions of AR 15 variants) are some of the most numerous rifles sold on the civilian market. Like all rifles, they make up a rather small percentage of the weapons used in gun-related crimes (handguns utterly dominate that statistic), but because some of the crimes in which they are used are very high profile (school shootings and such), the perception that they are big factors in gun violence overall is strong but inaccurate.