It's well known that the BATF considers drilling the evil third hole to be conversion to a machine gun. I spoke recently with a legal expert who clarified that the reason is that while drilling a hole is not considered "readily modified", dropping in parts that rely on such a hole is, and any gun that can have the parts dropped in and fire FA is, legally, an MG. I didn't realize until after we hung up that the hole alone is not enough; there also needs to be a slot cut into the receiver guide rail. Almost nobody mentions this slot, so I wonder if anyone here knows - is it too considered an MG conversion? The reason I ask is because I'm working on an invention that wouldn't need any additional holes (and my consultant, who didn't come cheap so I'm not asking them again, advised that simply drilling a hole in a different spot is ok), but it would need that slot.