The 5.56 or 223 will have a multitude of quality parts available, the 6.5 Grendel and in case you look to 6MM share a common requirement to research your build and make sure you adhere to procedures used by successful builders and manufacturers.most likely 2 223s but maybe a 6.5 Grendel
I will say the Aero Precision M4E I got in 6MM ARC is a fine rifle, I bought a factory assembled upper.