So I did order a used upper receiver that was supposed to be taken off an M4 from Shark Arms. It has the correct color and forge markings. No cracks or anything. Very clean. Built an upper for my son. No problems with that. But based on the thread at AR15.com it looks like the out of spec issue is a big deal. Even if you shim the rear sight, it will not lay flat when you flip it to change the aperture.
I am cancelling the order.
I am cancelling the order.