This is partially true and partially misunderstanding how this kind of thing works. While a relatively basic component like a shell casing would be easy enough to whip up for an experienced CNC operator, especially given an existing item to work from, laying out an entire device, such as a gun, typically requires man-years of effort. If you ask even a highly capable designer to create an arbitrary item you need, it could take them weeks to months to get you a result, and I wouldn’t expect that result to be comparable to the current state of the art. Schematics of working devices would massively jump-start this sort of thing, enabling you to start producing things almost immediately (given tools and components).
I could easily see a system where machinists are assigned to replicate the design of various items you might need, and over time produce their own schematics based on time, skill level, and schematic difficulty level.
There’s also a side issue here, while you can use a very capable cnc machine to mill out a part, it is incredibly resource intensive to do so, since that kind of device takes a block of the material as large as the largest dimensions of the item in question and simply cuts away everything that is not part of the desired item. A more mature manufacturing process wastes a tiny fraction of the material as a prototyping system by breaking it down into a larger number of steps that produces components that compose the desired item in just the right way. Also the cnc approach ties up one massively complex/expensive machine for the entire time it takes to make an item, whereas a mature production system spreads that time across a large number of relatively simple/cheap machines that do one thing but do it very well. the throughput of the mature manufacturing process is orders of magnitude higher.
In game terms, I wouldn’t want to get into specific machines, but rather pretend they’re somewhat interchangeable and just track basic facts about them like what material they work with and how complex they are, so the basic alternatives you’d have are:
“prototyping machines” that can perform the entire process start to finish, but waste a lot of materials and have low throughput, but also low to nonexistent setup time.
“assembly line machines” that perform just one step of a process, meaning you’d need to devote a number of them to producing one product. they would use materials much more efficiently, and produce items much faster, but the whole group of machines needs some amount of time to be configured to make something.
’'machine shop production" this is craftsmen using flexible machine tools, it would be even slower than the prototyping machines, but would tend to waste less material since the manufacturing process can be more flexible. setup time would also be intermediate. You could also throw a flexible number of craftsmen and tools at various projects.
There might be other categories, but you get the picture.
If you’re trying to have a faction design an item from scratch, they need either the right prototyping machines or a generic machine shop to work on, which would tie up the machines and consume materials until the design is complete.
If you need just one of something for some reason, you might be able to get a working prototype faster by cutting corners, but have to redo part of the design process to make it work with mass manufacturing.
