I didn't either state or imply that any outside vendor supplied parts weren't US made. I will note that it makes little commercial sense to create production facilities that already exist elsewhere and make quality products. There are a few builders, like LMT & Daniel Defense, that do special projects and do make more parts than many others. And charge accordingly.

Example: if you look about, you can find a list of forge marks of various forging companies that have made the raw forgings for AR receivers. The count is somewhere in the 20's, I don't feel like looking for my list. This doesn't mean that they're all currently forging receivers. Some only made receiver forgings under contract to either Colt or FN. Others apparently dropped out of the business. Back in 2012, there were about 5-6 forges (outside any proprietary forging facilities) making the raw receiver forgings. We had Colt 6920s that had lowers forged by a variety of forging companies, some of which are listed below.

Forge/hall marks include: stylized head of a cardinal (bird)-Cardinal Forge, hollow square-BAFE (think I got the letters right), there's also a broken A for Anchor Forge. All produce outstanding raw forgings.

Bolt carriers are also a foundation part made by a variety of manufacturers. The vast majority meet military specs. At least in the important areas. Some makers don't hard chrome the bolt recess on some of their products to reduce prices.

Note about QC inspections: the OCD inspections that got written into the military specifications were from Fairchild Industries who developed the design and began production. Fairchild was in the aviation business and on aircraft parts, you can trace every single person who performed any kind of work on any part. They also test each and every part to make sure it meets specs. In most industries, except for critical parts, QC testing is done on random samples unless problems are noted.

Also, the design originated in the 1950s. The QC on materials has improved exponentially since then. Lot to lot variation in materials has been greatly reduced. As has the tolerances produced by CNC machining.

ARs are kinda like cars. There are Buggattis, Rolls-Royces, Mercedes and down at the bottom, Yugos, Checkas and a couple of others. Most of us are happy with something in the equivalent of Chevy/Ford.

Do you really need a barrel from 4150? Not really, unless you're doing full auto. A hard chrome finish bore/chamber is really worth the extra $40 or so. Per the now retired CO of the USAMTU, the melonite coatings don't stand up to rapid fire. Neither does a plain finish bore, we'd destroy them in one CQB training rotation.