These are centuries-old objects. Manufacturing technology and materials science have advanced nearly 100 years since Ma Deuce first rolled off the line. Society didn't get dumber, and manufacturing has only gotten more accessible.
Just look at the current state of 3D printed firearms: they're completely useful and viable. CNC machining has never been cheaper or easier to do.
Evidence against the point above that it's trivial to replace professionally manufactured small arms.
I wasn’t aware of this. Do you have a good source where I might learn more?
It's similar to services: you can your own email server, but it's much more efficient to use some vendor and let them do it for you. But if all vendors cut you off, it won't be impossible for you to host your down email, it'll just be less efficient.
Maybe I'm overestimating the difficulty of making guns. But I'm aware of zero conflicts in which small arms were manufactured in situ. Even in e.g. Myanmar/Burma. The fact that even remote conflicts go through the trouble of importing arms suggests this might be more difficult than you suggest.