As I understand it, you'd need an
open bolt weapon to have something that's "readily converted" to full auto. As you'll note from the wiki entry, those haven't been legal in some time. Wiki's
National Firearms Act entry provides a little more clarification
In any case, it takes more than grinding down a sear to make a useful (read:
selective fire) automatic weapon that's not likely to just blow up in your face. I'd suggest the latter part is what you're likely to get if you take advice on modifying firearms from kids on youtube.