Small caliber, lots of plastic, short lifespan, unreliable.

Most serious 3D printed guns have at least a metal barrel. Often, 3D printed guns are just a lower receiver, that is the part you hold in your hand, the parts that actually fire the bullets (barrel, pin, slide, etc...) are bought off the shelf from real gun manufacturers.

This is a workaround for some laws that considers gun parts to not be a gun. For example, outside of a gun, a barrel is just a metal pipe and can be traded freely. The part that makes a gun a gun is the lower receiver, and you can 3D print that in plastic and still get good performance. In fact, Glock makes this part in plastic and these are some of the most popular and proven guns in the world.

It wasn't for the parts that go boom, it was custom ergonomic parts for a legally purchased, factory made firearm - the bits of the gun that you hold. Those are under less stress, so with the right filament and infill settings it holds up well enough. And when it starts to crack I can print another for $1 instead of buying one for $30-60. Plus I can change the shape and size to fit me better than an off the shelf part.

I had a buddy in high school who did competitive shooting, he did the same thing by carving these parts from wood, now it can be done for a fraction if the cost. Check out Olympic shooting pictures for an idea of the parts I'm talking about.

Most of a gun doesn't see that stress. put a barrel on a plastic gun and something metal behind and good enough for most needs.

Most "ghost guns" use factory made parts for pressure bearing components.

US is a rare country that treats only serialized parts as "guns" and don't regulate actual gun parts. This allow Americans to trivially "build guns at home", but easd of actually building guns is extremely exaggerated.

Also, there's little in 3D printers that actually makes it easy to build gun parts. It's all cosmetics. Real illegal ghost guns are always made with plumbing supplies.

> replacement parts for a gun

Perhaps the grip.