3DPrinting

What people are doing with "3D printed" guns is printing the receiver or frame components that are otherwise serial numbered and federally regulated, and populating those with metal barrels, slides, upper receivers, trigger assemblies, pins, springs, etc., as appropriate from the genuine item. These can be quite functional and durable, because the majority of the gun is, in fact, still made from "real" gun parts.

Clever individuals have gotten quite far in managing to print most of the required components, but several critical parts simply can't be made with consumer level printing technology. At present it is impossible to fully print a gun out of plastic and actually have it work.

The way federal law works, the ATF has identified and decided what constitutes the minimum identifiable major "gun part" of a given model of firearm, which is the part that must bear the serial number and is the component that cannot be sold without a background check through an FFL. For the Armalite platform, for instance, it is the lower receiver which is a component that can be 3D printed. The upper receiver is not a regulated part. For many polymer framed pistols like Glocks, the grip housing and frame is the FFL component. These can typically be 3D printed as well. But some guns, like the PTR/HK 91 and Sig P250 it's not the frame, it's the trigger assembly that's the FFL item. You can't effectively 3D print one of those -- although you could probably manufacture one with a milling machine pretty easily.

Even in one shot the 3d printed gun will explode. The cartridge is just a container for the gun powder, not the explosion. Real guns have a chamber that contains this explosive pressure.

3d printed guns are nowhere near strong enough to contain this pressure and when the gun fires the bullet is flung harmlessly in some random direction. Since there is almost no energy imparted into the bullet it doesn't have any power or lethality, heck the shrapnel from the casing is literally more deadly for the shooter than any bullet towards the shootee.

Heck a 3d printed gun can even fire a bullet at all. Plastic is not rigid enough to detonate the primer and set the round off. You can literally fry bullets in a cheap metal pot and when they explode they won't even go through the pot.

The only way you could make a 3d printed gun work is by incorporating tons of other metal parts, at which point it isn't a "3d printed gun". Search up pipe shotguns. They can be made with a handful parts from home Depot and only require 1 or 2 tools at home (only 1 if you get them cut at home Depot). Far more effective and actually deadly, even used by guerilla forces against imperial Japan in the Philippines.

depends on the design, as well as the capability of the printer.

DMLS is capable of producing basically anything you can think of in metal. FDM or resin, or whatever... you're printing the frame.

the DEFCAD design, specifically, you're printing the AR lower receiver- which for some stupid reasons is designated as the "firearm" as far as laws and regulations go. So you can print the lower and buy the rest in cash as parts.that said, the only real function the lower serves in an AR is holding the magazine in place, so it's not really subjected to anything that's going to break it.

Incidentally, $40 at a big box store and a lot of TLC with a dremmel can produce a passable SMG. in fact... many of the ww2 era machine guns were designed to be made in factories that used to turn out plumbing parts. (because this reduced the amount of time and materials spent on retooling the production lines.)

Fully 3d printed ones, yes. But you can print all the plastic parts of a Glock, buy a kit of parts that don't require any verification at all and assemble a fully working one that is about as good as a genuine glock.
Or go a bit further with the FGC-9 or countless other similar things. The fewest actual gun parts used in successful firearms are in .22lr pepperboxes which use only barrel liners.

Here in Finland, I couldn't do any of that, because barrels, liners, trigger assemblies, magazines, ammo, they all require a background check and having a license to own a firearm. As would those printed Glock upper/lower parts, if I had access to the kits making them illegal to own.
Instead of, you know, the 3d printer?

If it's only 3d printed plastic, yes. Most "3d printed guns" are like Glocks. Metal for the important bits, plastic for everything else.

I was watching some videos, they look pretty sturdy.

Apparently rebels in Myanmar are even using them.