Yes many companies get the Raw forgings for lowers from a limited number of providers. However the raw forging doesn't mean much when it comes to the spec of a part. How is it finished is more important for the lower.
As for your other point, some companies get parts from the same provider as other companies. However what you fail to take into account is the requirements each company has. Company A will ask for a higher level of QC while company B will accept whatever they are sent.
You obviously have little knowledge of manufacturing processes. The same vendor is not going to make a run of just a few hundred or thousand of the same part to one quality level and then make another run of the same part to a different one when starting with the same forging. It would actually cost them more money to do it that way.
The final milling of these parts are all done by CNC machines and it takes no longer to produce the part to this minor spec than another. However changing the setups is a very time consuming process that means you're making nothing during that time. Thus losing money.
Were there any differences in the forgings to begin with, then you'd have a issue.
And as far as the "small parts", springs, plungers, detents, hammers, triggers etc... Most of those are made offshore. And I mean the vast majority of them... That doesn't mean they're bad parts by any means. The same with stocks and fore ends... very few of them are true "mil-spec". Does it matter? So long as they're of good quality, NO.
And most of the time the difference between "mil-spec" and not means nothing more than the same testing procedures were not followed. Per unit testing is required for most mil-spec parts, whereas "batch testing" has been the world wide manufacturing standard for many many decades. It's less expensive, the customer reaps the benefits of those savings. Uncle sam is spending YOUR money, they don't much give a crap how much extra it costs.
I won't argue the point any further. You think you know what you're talking about because you've read it somewhere. I've been in the supplier end of this business (manufacturing) and I'm well aware of what's what and who's buying what parts from whom.