A hair trigger is just a trigger with an extremely light pull. You barely have to tough it for the gun to go off. However, having a hair trigger does not miraculously transform a gun from semi-auto to full-auto.
If you want a fully automatic gun, you'll have to buy one. First, you'll have to be lucky enough to live in one of the 34 states that allow you to own one. Second, you'll have to fill out some paperwork and send $200 to the BATFE. Third, you'll have to find one for sale.
The third point tends to be a deal breaker. Machine guns are EXPENSIVE! Because of laws passed in 1986, there are no new guns available to civilians. The guns for sale today are the same ones that were registered in May of 1986. Since the supply is so very limited (and getting smaller every year), prices for the crappiest, least desirable machine gun you can find, in really bad condition, start around $3,000. Desirable guns, in very-used condition, can run $30,000 - $50,000. Exotic guns are over $300,000.
There are things you can do to your semi that make it "pretend" to be a full. Look into bump firing. Also, some companies make trigger modifications that emulate bump firing without the need to learn a technique.