Technology

Depends on the type of 3D printer.

Fused Deposition Modeling, the standard "filament" 3D printing everyone thinks about when hearing the word 3D printing prints with plastics - of some sort. All of them,to a certain degree.

There are incredibly sturdy options nowadays, which include carbon fibers, but in the end the adhesions between layers will always be an issue. There are also options to print a cast for a mold and some funny techniques where you print a model with a specialised filament that is half plastic,half metal powder and send it to a company which "burn off" the plastic part, replace it with metal and send you back an (almost as sturdy as a cast) part that is fully metal.

Resin based printing is also a thing but not nearly as sturdy as FDM.

Last but not least there is metal powder based SLS(Selective Laser Sintering), but that does not produce those sturdy parts everyone thinks of, is extremely sensitive/requires a lot of knowledge and lastly money - these printers start around 20k for the better models.

In terms of additive manufacturing people are able to print non load bearing gun parts. Maybe even sturdier than before. And easier. (A 400$ printer nowadays does what a 1300$ printer did a year ago and a 15000$ printer did 15 years ago. But for everything load/pressure/shock bearing, like a barrel, spring assembly,firing pin,etc. will still need to be from pure metal. So people would still need to improvise these,most important, parts.

BUT: There are also self-built CNC machines. MPCNC, etc. are a thing, and more advanced projects for around 2500-3000$ omwards can easily achieve a level of precision on steel that is more than sufficient for an all metal ghost gun and close to what industrial guns makers in WW1, maybe even WW2, achieved.

It's currently really the golden age of home manufacturing.

Metals, catbon fiber but I heard thise nozzles wear out fast. One printer i saw used lasers to weld metal powder into shapes layer by layer.

Carbon fibre is a big one.