It is illegal to construct or possess a gun that is "readily convertible" into an automatic (more than one round per trigger pull); any such device is classified as an NFA weapon (machine gun). A common example is a rifle that can, with only a small amount of milling or other work, accept an autosear.
This is untrue. An AR-15 can be made to fire more than one round simply by removing (or incorrectly installing) a very tiny spring. An AK-style rifle can be made full auto by removing a small amount of metal in the right place, no other parts needed. And an SKS can be made to empty its magazine without any trigger pull whatsoever by simply jamming one piece of its mechanism into a certain position with a tiny wad of paper or something. Those are just three examples.
So then, I could legally have in my possession an AR-15, an AKM, and an SKS. Right next to those rifles I could have a pair of wire cutters, and small file, and a scrap of paper. I would not be breaking the law.
The thing you're likely thinking of is called "constructive intent". If I have all of the above in my possession with the intent of violating the NFA, then I'm breaking the law. Of course, it's up to the BATFE to decide if my intent was worthy of charging me with a crime, and then the court to decide if that crime was committed.