You're going to be up against ITAR for a few key components - startup cost for lathe and milling capability to make those parts yourself is going to be considerable (but can be recooped - less if you can buy used and find people with enough machining expertise to work around having worn equipment). Another issue will be sourcing raw materials (with a good enough papertrail that you can be really confident you're actually putting in the metalworking costs on metallurgically correct material), but that's a cost issue as much as anything else.
The number of 'manufacturers' in the US isn't as impressive if you narrow it down to a few really critical components (most barrels only come from 25 or so sources' bolts, carriers, and receiver extensions about the same; lower parts despite their comparative simplicity are economically sourced from one of just a few places), but there are still many different places.
Unless you have access to around $400,000 USD in combined capital, it's going to be extremely hard. That much with the right technical skill (machining, CAD/CAM, metrology, quality assurance, assembly; not to mention business and staffing, as well as surface coating firms to handle anodize/parkerize) and it's actually fairly straightforward if you have the right drawings to work off of and aren't having to worry about legal concerns.
FWIW, the machine shop I work at has the capability to make complete AR's if we source barrel blanks and outsource surface treatment (for us, OSHA concerns on any surface treatment); but we make other things instead because of the better profit margin. We make satellite parts regularly (complex, high precision, tight tolerance stuff), and still our most lucrative stuff is elsewhere.
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