There are two possible answers to this question.
1) There are full-auto trigger packs, "Drop-In-Auto-Sears" (DIAS), "Lightning-Links" and similar assemblies which have been registered -- themselves -- as machine guns (prior to May 1986) which can be installed in a legal "Title I" firearm allowing it to run full-auto. If you own one of these transferable items, you can swap them between guns. Some of these, especially the H&K trigger packs, can run several different models of a series of rifle, so -- as long as you don't mind buying several moderately expensive semi-auto rifles -- you can sort of have a whole stable of machine guns with just one registered piece/assembly.
2) Adding parts, filing things, bending springs, adding a piece of cardboard or wire, or otherwise fooling with a gun to make it run full-auto is DEFINITELY completely illegal, and usually a really dangerous idea. On the one hand, you are absolutely making an unregistered Title II firearm (10 years in federal prison, $250,000 fine). On the other hand, you're likely making your rifle slam-fire and/or go "runaway" and this will destroy the gun and hurt you sooner or later.