Buying fully-auto firearms is difficult and expensive. You are required to purchase permits and become subject to ATF inspections. Most people wanting full-auto guns would also like the ATF to leave them alone.
There is a rising trend in the firearm industry to enjoy full auto without all the hassle. Air rifles provide a cheap, easy way to play around with full-auto. I am currently testing full-auto Air rifles from pyramid air. They are a lot of fun and start at only $100!
Here is a review video from 22 plinkster, one of the biggest names on YouTube in the firearm industry.