[FelixP]

The short answer is that it is very hard to acquire fully-automatic weapons in the US today. Aside from the fact that anything available for sale to civilians will be both prohibitively expensive (1-2 orders of magnitude more expensive than a comparable semi-automatic weapon) and most likely an antique (only weapons manufactured before 1986 are available to civilians), the acquisition process itself is pretty arduous (e.g. FBI background investigation, signoff from head of local law enforcement, etc.). From my understanding, these weapons are almost never used to commit crimes.

The real danger are handguns - something like 3/4s of gun deaths are from handguns.