Basically, the majority of the brand names you encounter are assembly warehouses for parts made in machine shops that sell mostly to the assemblers. Nobody has the industrial capacity to make everything components-wise. Some name-brand businesses make their own barrels, or their own receivers, or one or two major assemblies, then source out the rest.
There are the following component makers, some of whom are sole source:
Barrel makers
Barrel extension makers
Barrel nut makers
Bolt Carrier and Bolt makers
Carrier key makers
Hard-chroming shops that precision electro-plate the inside of the carrier and carrier key
Cam pin makers
Firing Pin makers
Fasteners are common Grade 8
Extractor makers
Ejector pin and other small part makers
Spring makers (recoil, extractor, ejector, trigger, hammer, disconnector, mag catch, bolt catch plunger, weld spring, detent, snap ring, ejection port doors, front sights..)
Trigger, hammer, disconnector makers (usually MIM)
Pin & detent makers (milspec calls for cadmium-plated detents, most of the civvy market uses zinc-plated detents)
Selector, bolt catch, mag catch, mag release button
Aluminum forgers make the raw blanks on forge plates
Machine shops do the CNC finish work on the forgings
Anodizers do the Type III hard-coat on the receivers
Gas tube makers
Front Sight Base forgings are steel
FSB's are then CNC finish-machined somewhere else
Quality extension tubes are forged, not machined from billet, then anodized
Buttstock and pistol grip screws are made by shops that specialize in screws
Forward Assists are made by one machine shop
Sight components are another industry
Furniture is a whole separate field, where some are great with glass-filled nylon, and others aren't.
The reality is that your BCG comes from one of a very few set of sources, and many of the sub components are from one or two places. Yes, the assemblers you associate with a particular component group do have the final manufacturer point of quality control/assurance, but everything is mostly ISO-9001 now, so very seldom do parts make it out of the shops if they are out of whatever the specified dimensions and material specs are. Just keep in mind that those industry specs aren't the Mil-spec.
