The home modification alluded to is illegal and there is no possible way to make it legal - i.e. no tax stamp available for guns made after 1986, like others have said. Also, I doubt that it would work, other than perhaps an occasional double before it jams.
Unfortunately, legal machine guns start at around $3,500, excluding transfer costs and $200 stamp tax. There's no way around that.
If you want something that simulates full-auto without going to jail and without ruining a gun, look into building a semi-auto AR-15 and get a Slide-Fire stock. All in, it should cost you about $1,100.