This is possible because the chamber and barrel are all of one interchangeable piece on a Liberator. You simply make one in your desired caliber - they made one in 5.7, slipped it on the end of the mechanism and pulled the trigger (remotely). Think of it like a Thompson/Center Contender, where you only change the barrel to change the caliber of the weapon.
If you are planning on murdering someone you actually know, no, a Liberator (indeed, the murder itself) is not a good idea! They’d already be looking at you anyway. You are forgetting one thing about the raw ‘traceability’ of a Liberator, though - it’s not very, in and of itself. While the authorities (let’s assume we’ve entered a period where the government is like that in Escape From LA) can go around to every home improvement store in an area and get video of anyone buying the materials from which a zip gun could be constructed and then interview them, it is far more difficult to track a Liberator back to any person or groups of persons, especially considering that you can order filament and the rest of the materials from overseas cheaply and easily. If you are playing the Polish Resistance to the government’s German Occupying Force to ‘borrow’ better weapons, that’s going to be quite important.
Also, if you live someplace where you are not allowed to carry a firearm for self defense let alone use one, a Liberator might be a good idea. Thug tries to kill you, you shoot them then you can quickly dispose of the weapon without fear of it being traced back and without any great financial loss on your part. This, by the way, is what has police in the UK all up in arms at the moment. In neither the Resistance case or the self-defense one would the authorities be pointed in your direction and leaving the Liberator at or near the scene would not give them a clue as to your identity. In the murder case, they’d already know who you were, of course, so it’d not be a help (nor should it be).
The Liberator is not designed to be a proof of concept weapon, it’s a last ditch/last resort weapon - like the WW2 FP-45 it is named after and it is not intended to be used as a service weapon.
Think of it this way - the Liberator, as it stands in the designed caliber, is designed to be a single shot disposable pistol, and it does that very well without blowing up. Much like reloading other single-use consumer devices, subsequent usage after the initial one may or may not work. But as designed, it will NOT blow up on its initial use.
Basically any imperfection in the path of the projectile before it exits the barrel could easily cause over pressurization that would result in a catastrophic failure. This is why most if not all firearms company perform a test fire before shipment. I believe many states require this.
It seems that you have more knowledge and faith in the predictability of the strength of the output from a 3D Printer than I do. The output of different types of printers and the choices in filament obviously could result in large variations in quality. Even the settings chosen for the same printer could cause differences in the strength of the finished product. I don't doubt that many of the prototypes did not blow up on their initial use. But I know of guns whose primary components were made completely of steel which have had catastrophic failures.
I understand the reasoning that you have presented. One advantage that it would seem to be fairly easy to completely destroy a printed firearm. In most situations I disagree that tracing back a recovered more traditional homemade firearm would be more difficult than finding the person who printed a gun. Currently the availability of parts that could be made into a “zip gun” is almost limitless. The availability of materials and 3D printers capable of producing a gun is far less common. I believe this is true both in the United States and abroad.
I do thank you for the informative discussion. Personally, I won't be switching from the handguns that I am licensed to carry anytime soon.