Printing barrels and FCUs -- the fire control unit, which is the only thing tracked and serialized in a gun at least in the US -- is more difficult but not impossible. Actually, building a functional FCU that can strike a bullet primer, or a barrel that can be used once is not difficult at all and if you look around you can find videos of people that have tested that with a mixture of 3d printing and rudimentary metal working skills. The major issues on designing those parts are reliability and safety. In the Philippines there is a full bootleg gunsmith industry dedicated to build illegal guns that match commercial ones in those aspects too.
Sadly, instead of having better laws we get fallacy rhetoric by people who probably have never touched, much less fired a gun in their lives.
Why is this the litmus test for being qualified to write gun legislation? Do we also expect our lawmakers to have tried heroin or downloaded child porn so that they can regulate those activities?
The government allows private ownership of automatic weapons, but hasn't issued any new tax stamps for 50 years. You can convert any semiauto gun into a full-auto gun for a few cents of 3D printed parts (or a rubber band). The hysteria over "assault weapons" basically outlawed guns that _looked_ scary, while not meaningfully making anyone safer.
I think yes, it is reasonable for Congresspeople to fire a gun before they legislate on it, because otherwise they are incapable of writing good laws.
Good gun regulation in the US would probably look like car insurance, where gun owners need to register and insure their weapons against the possibility of crimes being committed with them. There are so many guns compared to the amount of gun crime that it would probably not end up terribly expensive, especially if you own a gun safe.