Most of the 3D printers currently in use bulid up the part with layers of plastic material. ABS is a popular material. Complex and hollow forms require a supporting material, which the printer also lays down as needed. When the part is finished, the supporting material is either mechanically stripped away, or dissolved.
Thanks for your comments, AB. I still find it hard to grasp, though. Building up layers by “spraying” or depositing (printing?) liquid-ABS is a far cry from building up layers of S35VN Stainless Steel, for example....wouldn’t you say? Currently, they may be able to laser-trim simple solid parts from lumps of steel, but I believe something as complicated as a rifled gun barrel is a bit of a leap into the future at this point. Being a retired engineer, I have great faith that it will be accomplished. But I wouldn’t want to venture a date as to when.