DPMS chambers their barrels to the 5.56NATO dimension. However, their tubes are marked .223. I found this data from a very reputable website, but they MIGHT chamber their bull-barrels to .223rem??
I have one of the DPMS CMP uppers and it's diet consists of Santa Barbara ss109, South African surplus (for practice) and Black Hills (for business).
I have YET to have a problem. This is one outstanding upper.
Hope this helps?