Once a machine gun, always a machine gun.
On an AR-15 receiver this is true. There are two modifications that need to be made to install the full-auto parts. You will note the extra pin above the selector switch in the picture posted earlier. That's one modification. Once this hole has been drilled to accept the auto disconnect, you own a machine gun by BATF rules even if there are no full-auto parts installed.
Almost all semi-auto firearms require some modification to the receiver to install full auto parts.
Modifications that allow a semi-auto to fire full-auto (like triger sear adjustments) without installing the full auto parts are often dangerous and can results in the injury or death of the shooter. That's why it's so important to take a gun out of service if it doubles or triples on a trigger pull. This is commonally referred to as slam fire.
The only guns that were built for slam fire are sub-machine guns that use pistol ammo. They generally fired from an open bolt.
Personally, I don't care very much for small full auto weapons. They just don't do a very good job. Semi-auto weapons can fire so fast that full-auto is not needed in almost all situations. I used a full auto m-16 in the military. I was not impressed. Select fire was much better. I was lucky, I never had to enter a close range fire fight were full auto would have been usefull.
Still, for those who want a full auto weapon, there is a legal path to ownership.