You clearly didn’t read anything I said after that first line but this is just too fun:

A CNC can make a gun from start to finish that you wouldn’t be worried about blowing your fingers off when you pulled the trigger, without adding outside hardware to.

Mostly tells me you have no idea what is inside of a firearm outside of “magic”.

Over simplifying, but modern firearms generally consist of

1. A frame: This is usually a mix of plastic and metal for comfort, weight, and heat dissipation reasons. But there is nothing saying you can’t have an all metal frame. Some parts will receive load, some parts are purely cosmetic/ergonomic. This is demonstrably not a problem for either 3d printer or mill.
2. Lots of pins and levers: These are the mechanisms that actually make the gun go bang when you pull the trigger. And, depending on how spicy you want to get, continue to go bang while you hold down the trigger. Again, some parts are stressed, some aren’t. There are ways to do all plastic that should last at least a few hundred rounds. The issue is more one of tolerances. Getting a 3d printer to be able to print with that level of precision is more work than people want to put in. Similarly, getting a mill capable of that level of precision is also a mother (Adam Savage of Will Smith’s Tested fame has a lot of videos where he is basically tackling this challenge for different projects). But also? These are the parts that “wear out” and are completely uncontrolled and can be bought online trivially. So why manufacture this yourself in the first place?
3. Springs. Good fricking luck milling your own springs with the required properties. Also good fricking luck printing your own springs. That said, I actually watched a REALLY cool video where someone proof of concept made a gun out of 3d printable leaf springs and while the guy doing it was a complete and utter dipshit, it was a work of engineering beauty that you couldn’t pay me enough to hold in my hand… But, again, these are trivially easy to buy online so why bother?
4. The last major part in this oversimplified breakdown is the barrel (and how you seal it but I am just going to include that here). These are the parts that experience the most forces and heat when the gun is fired since that is the part where the bang happens. Everything else you can probably get away with some cheap mild steel. That? You need some good stuff otherwise you are going to learn just how weak that cheap stock was. You CAN mill chrome moly and the like but that is a much bigger challenge than cutting through cheap steel like it is butter and amplifies all of the above tolerance issues. I will outright say that you can’t print this in plastic even though I have seen in person demonstrations of proof of concepts of that.

And the cheap steel problem? Go look up pictures of the “custom” guns used in conflicts like The Troubles. A lot have cracked or split barrels and the like because of that exact issue.

Which, like I said in the comment you refused to read, is why you just buy the hard parts online or in cash at your local gun store (… might need to go to a specialty shop instead of a Walmart). Because they are not controlled since they generally need to get replaced eventually anyway.

At which point? Mills and Printers both work great. It is just that the former needs a lot more machinist skill to be done. Whereas the latter is just downloading an STL.