Drew, the mil spec of the AR is a base design. Different companies go above and beyond to different extents, some to make a good gun, others to save some $$.
This can range from a different twist in the rifling, to a different method of testing the pieces of the gun. Some companies batch test, others sue individual testing of the parts.
My next AR will be a BCM, the one I got is.a bushmaster LEO model.
