Gun fail

From a buddy of mine. Addresses you question:

As you know, the original weapon was created by and for the military and given the designation M-16. Later there was a demand for a civilian legal version of the M-16 and was created strictly for civilian use in a civilian legal semiautomatic format. The designation of AR-15 was come up with which means semi-auto and makes the weapon by definition a semiautomatic.
There are how ever people that did modify the guts of the receivers back in the day so their AR-15 would fire full-auto. Now the modern receivers today are built in such a way that the inside is cast as to not allow those modifications to be completed without first having to do some milling work to the inside of the receiver (not easy to do).
There are some trigger kits available out there called "binary triggers" that can be installed fairly easily on an AR-15 that allow the gun to have the selector switch to be placed in the full-auto spot and when done so the gun will fire when the trigger is pulled and fir
e again when the trigger is released. Hence if you pull the trigger very fast you got yourself a basic full auto knock off that is 100%, by ATF definition of what a machine gun is, perfectly legal. But for it to work correctly you must also purchase a full-auto bolt carrier group and replace the original AR-15 semiautomatic bolt carrier group. Binary trigger and new bcg will run you about $600-$700.