2. Regarding airsoft rifles, the problem was that some of them were so similar to the real thing that you could take the automatic airsoft lower and put a real firearm upper on it and conceivably have an unregistered machinegun. To the extent that can happen, it is a concern for ATF.
Even with a lower that perfectly matched a machinegun lower, there are still some minor differences (often bolt carrier and firing pin flange diameter) that would make most semi-auto uppers on a machinegun lower slightly less reliable than an actual M4.
Hold up, aren't these airsoft guns battery powered? How would this even work?... I am thoroughly confused now....