not that im agreeing or disagreeing with the whole mechanic thing, but, the employees that build them are NOT gunsmiths, they are ASSEMBLY TECHS. they run off an assembly line. lots of videos to watch on the process that the big guys use.
There are some big name manufacturers that have pulled their assembly videos due to the criticism received for doing things improperly. The majority of AR’s you come across at your LGS are not going to be well made. That’s due to over saturated market and current high demand.