A weapon can be made automatic, with an appropriate fire control group that is illegal to manufacture or possess unless it was manufactured before 1986, or you're LE/MIL or have a Class 3 FFL. Talking about "urban legends", one of them is that you just have to file down a part here or there or that a semi-automatic firearm can be magically turned into an automatic firearm easily without significant modification and additional parts. If you could convert a semi-auto to automatic, you'd also have the tools to fabricate your own automatic receiver yourself, no conversion needed.