I never understood the purpose of the “undetectable” firearms law anyway. The bullets are still detectable.
I always understood undetectable to be mean not detectable by most metal detectors which are magnetometers I believe. Tempered steel is used in most guns due to it being the cheapest material with the required properties. I do not know the magnetic properties of most ammo, but believe if you were willing to spend enough money you could build a gun that was both reliable and did not interact with a magnetic field.