Roaming the Web
With the vagueness of this topic it's probably best to set up some boundary conditions. It often makes sense to me to look at the extremes to try and decide what should be allowed.
weapon manufacturing that is to say military-grade weaponry
This can range from manufacturing tools that can be used as weapons (e.g. knives), to non lethal weapons (e.g. tear gas), to weapons of mass destruction (e.g. nuclear weapons) (all of these are used by the military in some form or another and therefore could arguably be "military grade"). It also can range from tiny scale (e.g. one time project) to massive scale (e.g. multinational corporation).
should only be allowed by the state
Regulation can range from requiring a nominal permit to manufacture weapons to a complete ban of civilian manufacturing for all purposes (including non-combat purposes such as education/research).
In my opinion, regulation should be constructed that prevents weapons from getting into the hands of people that current regulation is designed to prevent them from obtaining. This would likely result in laws on the more strict side of the above sliding scales, banning the manufacture of all regulated weapons by an individual unless they're granted special permission for education/research purposes (what weapons should be regulated is another topic entirely). The main problem with this approach is that its nearly impossible to enforce. Unlike with, say, counterfeit currency, there are a broad range of designs of weapons that are possible that will still be effective, making trying to regulate the tools used in the manufacturing process (e.g. 3D printing programs) pointless.
At least with this form of regulation, it would prevent the unauthorized manufacture and sale of homemade weapons (which there are probably laws on the books already depending on where you are), but in a future world where access to 3D printed guns are as simple as a trip to the library's 3D printer, the laws seem ineffective. I'm not really sure what the solution is here.