Competition

Collecting/re-enactment

Employment (No, those jobs aren't in the help wanted section of the newspaper)

There are relatively few legal full auto weapons in the US, as they cannot be manufactured for civilian sale (I forget the cutoff date, but it is many years ago that the manufacture and importation of F/A weapons for civilian sale was banned).

The people who legally own them are some of the most stable, sane people you would ever meet.