The differences amongst different brands are not much these days.
Yes, back 10 years ago it mattered more, and 15 years ago it was a big deal.
But in 2017, most manufacturers and their networks of subcontractors have perfected the art of mass producing AR. Even cheap AR works.
Yes, there are the "closer to mil spec " guns. Little things insisted by the military like MP, chromelining, certified steel.....chromelining of BGC interior. But the wide spread use of different technologies like nitriding of more available steel drops the cost and make mass production faster. The end products are just as useable for most applications.
When it comes to dimension and tolerance in the actual TDP, the TDP has been floating around and seen by many contractors who work for Colt and FN over the years. Yes, they cannot actually "copy" the TDP, but if you are an engineer you read through the TDP and you will get the jizz of it. The cat had been out of the bag for many years.