Humpty, this is a great topic - it applies not only to gunmaking but to everything technological and to scientific instruments as well. I think there might be some question about your assumption:
"By common understanding, low-tech is something that you can do at home. Rust-blueing is a good example. High-tech, by contrast, is something that requires lots of investment and collective effort. A billion-dollar lab, a crowd of highbrow researchers with scary-sounding degrees, who work for years and years and then come up with a new plastic fantastic or cryogenic treatment technology. Then, it takes a huge corporation with big production unit, armies of managers, accountants, marketers, etc., etc., to produce the equipment with which one could take this research and make a gun using its results."
If we look back in time even as far back as late Medieval or Renaissance times, going into the 16th and 17th centuries, a lot of superb mechanical devices were being made. These were all "leading-edge" for the period and what we would today call "high-tech." Making an astrolabe, for example, required highly specialized mathematical and astronomical knowledge, highly competent artisans and sources of quality metal. In the 17th and 18th c, the air pump was about as high-tech as any instrument or mechanical device in existence. It required large capital input, skilled artisans in many fields and they were expensive and limited to a few natural philosophers. All these devices stretched the limits of then-current technology, financing and trade skills.
In all historical contexts technology draws upon what is available at the time and despite the notion that these devices were crude or "home-made" they were certainly not - they required great skill to make and also a lot of money. They also required the efforts of more than one person, in general. There were no large factories, PhDs, subspecialty engineers, marketing departments and such. This in no way diminishes the "high-tech" quality of that product. Gun-making falls into the same category.
Home-made and Hand-made are very misunderstood descriptors. Any physical artifact is a material execution of a concept - how that execution occurs depends upon the time period and whenever a design change implementing new concepts is implemented it usually involves the best technology of the period. What was "high-tech" in the 17th, 18th or 19th century might have been supplanted by newer technologies and is now "low-tech" but not necessarily something that anyone could do at home.
The gun is simply another mechanical instrument and its manufacture and development parallels that of other devices. The modern gun draws upon many outside developments (as noted) including powder, optics, steel. It can still be made "by hand" and there are probably a few makers who can do that but in general a lot of automated machinery is used. Neither the machinery nor the hand labor makes the product low-tech or high-tech.
