I don't think that the developers purpose of these gns is to be undetectable, but to bypass gun registration.
However, if that is your goal, then you could just make plastic ammo. A plastic bullet may not be as heavy as a metal one, and the casing might be a bit dangerous, but the right plastic may make it passable.
Remember, right now the fabrication machines are in their infancy. Just look at how computers have evolved. If these things evolve like computers have, simple guns like these will be the least of your problems.
But, a more important development would be an assault rubber band gun, with a 30+ clip.