Given that most of us don’t always have perfect access to truly “genuine” mil-spec, mil-standard parts (those actually used in issued fighting tools), we’re shopping based on a little part trust and a bigger part knowledge. The trust part is accepting claims of “USGI-spec” parts actually being done to not only blueprint dimensions, but also made from the correct materials treated to the same processes. That’s receivers, bolts, and on down the list of the 100 or so parts that can make up an AR-15.