And even less of it is done by most AR-15 manufacturers. AR-15 rifles are made from vendor built components. The companies who build most of them are assemblers. You as the buyer have no idea who made what where, let alone who tested what. You put your faith in the finished product that comes out of the box. Why would a company invest in the tooling and machinery to produce components they can readily purchase on the open market from companies who are set up to make such parts? They don't.
Certs and testing. High pressure rounds, magnetic particle inspection, positive materials testing. Documentation is part of testing.
https://danieldefense.com/about-daniel-defense
So, uppers, lowers, barrel machine, testing.. All made right there to TDP specs (only Colt has the actual TDP, right?). They import some parts, but hey, look - it's not how you say it is. Great. The companies that do import parts inspect them for quality.
Hey everyone, even if you don't believe what I am saying, don't listen to this guy, because you do NOT have to have "no idea". Do the opposite.