1. As pointed out, pennies could not fit behind the firing pin, and if they could, would stop the hammer from hitting the pin and the weapon would not fire at all. So that theory is just bizarre.
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.