How about printing a plastic 38 revolver, and using a steel cylinder.
Anything that has a barrel and some kind of receiver durable enough that it won't blow up in your hand when you fired it would do.
And if it's only a one-shot (or disposable) that shouldn't be too hard considering the number of improvised (but deadly firearms) that can be fabricated even without a fancy 3D printer. A quick Google will net you dozens of designs - so it's not as if 3D printing brought a new capability to the table. It just promises to make it easier for the technically challenged - and possibly spawn a bigger US 'cottage' armament industry.
Not that you even need for it to be a traditional firearm. There are lots of other projectile weapons that don't depend on a charge to do their work. Although the phrase "zip gun" has gone out of vogue, it hasn't removed them from the street. ANd with some of the new materials they're being made from, they've gotten a lot deadlier.. A friend of mine who is with our State Police showed me a homebrew "gun" that could punch a triangular wedge of sharpened scrap metal through a half-inch piece of solid core plywood at 10 yards. With no flash and hardly any sound either - so it doesn't need a silencer or muzzle flash arrestor for 'stealth' use. This was the real deal - a genuine man-killer - and one nasty piece of hardware. Even had a pretty little silver cross amateurishly embedded in the grip.
The only thing that caused a drop in the zip gun's market share was the influx of dirt cheap "real guns" which started appearing (in the US) in earnest during the 80s.
Sad truth is, weapons acquisition is driven by a need - real or perceived - to have a fallback in an emergency.
The only way they're ever going to get a handle the number of guns on the street is if the powers that be finally start addressing the root causes behind it. Most people I know prefer not to own or carry a gun. But I'm seeing and hearing of more and more people who used to be adamantly opposed to "having a weapon in the house" now seriously considering getting one and learning how to use it.
Maybe the time has come to drop the glittering generalities and propaganda and start some genuine dialog about why this is happening?
Hello? Washington? State Governments? Is there anybody out there?