Automatic weapons have been heavily regulated since 1934.
You have to cough up a hefty amount of money and submit your name and fingerprints to the FBI for a lengthy criminal background check.
After that, you can count on spending several thousand dollars to get the cheapest of automatic weapons.
This includes M16's and AK47's (select fire).
I can recall only one time in the last 20 years or so that an automatic weapon was used in a crime.
So, with that in mind, why ban them?
Check out how much fun you can have shooting them:
http://www.tfproject.org/tfp/showthr...&threadid=3144
(moved to politics)